Epitaxial growth method of a zinc oxide based semiconductor layer, epitaxial crystal structure, epitaxial crystal growth apparatus, and semiconductor device

ABSTRACT

It is provided a hetero epitaxial growth method, a hetero epitaxial crystal structure, a hetero epitaxial growth apparatus and a semiconductor device, the method includes forming a buffer layer formed with the orienting film of an oxide, or the orienting film of nitride on a heterogeneous substrate; and performing crystal growth of a zinc oxide based semiconductor layer on the buffer layer using a halogenated group II metal and an oxygen material. It is provided a homo epitaxial growth method, a homo epitaxial crystal structure, a homo epitaxial growth apparatus and a semiconductor device, the homo epitaxial growth method includes introducing reactant gas mixing zinc containing gas and oxygen containing gas on a zinc oxide substrate; and performing crystal growth of a zinc oxide based semiconductor layer on the zinc oxide substrate.

CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. P2008-171575 filed on Jun. 30,2008, No. P2008-171610 filed on Jun. 30, 2008, No. P2008-334213 filed onDec. 26, 2008, No. P2009-048527 filed on Mar. 2, 2009, and No.P2009-123102 filed on May 21, 2009, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an epitaxial growth method, anepitaxial crystal structure, an epitaxial crystal growth apparatus, anda semiconductor device. More specifically, the present invention relatesto an epitaxial growth method, an epitaxial crystal structure, anepitaxial crystal growth apparatus, and a semiconductor device of zincoxide (hereinafter, it is referred to ZnO) based compound semiconductorcrystal of high quality, and a group V element doped ZnO basedsemiconductor, and a fabrication method and a fabricating apparatus forthe ZnO based semiconductor.

BACKGROUND ART

Since a ZnO crystal is a direct transition semiconductor whose bandgapis about 3.37 eV, its bound energy of the exciton which electrons andholes combined within the solid is as large as 60 meV, and it existsstably also at room temperature, the ZnO crystal is an affordable price,and its environmental load is also small. Therefore, the ZnO crystal isexpected as a light-emitting device from a blue light wavelength regionto an ultraviolet light wavelength region.

The ZnO crystal of a use is wide except a light-emitting device, andtherefore the application to a light-detecting element, a piezoelectricelement, a transistor, a transparent electrode, etc. is also expected.In order to use the ZnO crystal for the above-mentioned uses, theestablishment of ZnO crystal growth technology of high quality excellentin mass production nature is very important, and the doping technologyfor controlling the conductivity of a semiconductor is also important.

The following methods are known as a method of fabricating a ZnO basedsemiconductor of high quality. For example, in the MBE (Molecular BeamEpitaxy) method, the ZnO based semiconductor of high quality is grown upby supplying molecular beam of the zinc and the oxygen radical (plasma),and reacting the supplied zinc and oxygen on a growth substrate.Moreover, in the PLD (Pulsed Laser Deposition) method, the ZnO basedsemiconductor of high quality is grown up by illuminating the sinteredbody and crystal of the ZnO based semiconductor with a laser beam, anddepositing an evaporated ZnO based semiconductor on a growth substrate.

However, since a large area film formation is difficult and needs togrow in a vacuum when growing up the ZnO based semiconductor by the MBEtechnique and the PLD method mentioned above, there is a problem that itis difficult to mass-produce industrially.

Then, the method of growing up the ZnO based semiconductor by the MOCVD(Metal Organic Chemical Vapor Deposition) method widely used for thecrystal growth of a group III-V semiconductor is known as a fabricationmethod of the ZnO based semiconductor which does not need a high vacuum.In the MOCVD method, an organic metal including the zinc is decomposednear the substrate or on the substrate, finally the oxygen materialreacts to the metallic elements, and thereby the ZnO based semiconductoris grown up.

However, in the MOCVD method mentioned above, since the vapor pressureof the zinc which is a group II element is extremely high compared witha group III element, even if the zinc reaches the growth substrate, itis easy to separate from the growth substrate, under the hightemperature in which growth of high quality is possible. Accordingly,since the rate of the zinc which can be contributed to the growth of theZnO based semiconductor on the growth substrate is small, there is aproblem that the efficiency of material including the zinc is low.

Moreover, since carbon mixes into the ZnO based semiconductor by thehydrocarbon group generated when the organic metal material includingzinc is resolved, there is a problem that the growth of the ZnO basedsemiconductor which does not include carbon is difficult.

When growing up the ZnO based semiconductor by the VPE (Vapor PhaseEpitaxy) method, using a zinc single metal substance and oxygen material(for example, oxygen) including oxygen as a material is known as onemethod. However, since the equilibrium constant of the chemical reactionis large compared with the equilibrium constant of the group III-Vsemiconductor, and it is necessary to set up highly supplied partialpressure of the zinc with high vapor pressure for high temperaturegrowth as mentioned above, there is a problem that it is difficult tocontrol the reaction.

Then, the method using a zinc chloride and oxygen material as thealternative method in the case of growing up the ZnO based semiconductorby the VPE method is disclosed (for example, refer to Non PatentLiterature 1). In the fabrication method of the ZnO based semiconductoraccording to the Non Patent Literature 1, the ZnO based semiconductor isgrown up by installing the powder of zinc chloride in a reaction tube,carrying the zinc chloride which became steam by annealing by usingcarrier gas, and reacting the zinc chloride to oxygen.

The above-mentioned method is called an HVPE (Halide/Hydride Vapor PhaseEpitaxy) method using halogenated group II metal as a group II material.In addition, the HVPE method is known as a fabrication method for thegroup III-V semiconductor which fabricates a gallium nitride substrateetc. industrially by using a halogenide (chloride) for group IIImaterial, and using a hydride for group V material. In this HVPE method,the hot wall method, which makes high temperature not only the growthsubstrate and its circumference but also a quartz tube, is generallyused.

In the above mentioned Non Patent Literature 1, however, the fabricationmethod of the ZnO based semiconductor uses zinc chloride as zincmaterial and the zinc chloride is deliquescent. Furthermore, since thepurity of the zinc chloride which can be obtained easily is as low asabout 99.9% and the zinc chloride with high purity is expensive, the ZnObased semiconductor of high quality cannot be fabricated easily.

When using the ZnO substrate which is homogeneous material species as asubstrate for ZnO crystal growth, the substrate for the ZnO crystal isproduced with the hydrothermal synthesis method of the same method asthe production of quartz crystal. There is a problem that the impuritycontrol of the high level required in the semiconductor field is neededetc., and development of the hetero growth technology for growing up ona heterogeneous substrate is also needed.

When the nitride based semiconductor is used for the use of a LED (LightEmitting Diode) of which cost reduction is required in particular, asapphire substrate, a silicon carbide substrate, a silicon substrate,etc. are used instead of using the expensive substrate for homoepitaxial crystal growth.

When the high temperature growth is directly performed on the sapphiresubstrate by the MBE method or the MOCVD method, there is a phenomenonin which the ZnO does not grow, and it is a phenomenon not occurring inGaN.

It is because it is difficult to grow up the ZnO at high temperature asfor the element with low wettability on the substrate, such as Zn, ifthe wettability is not yet improved by forming the ZnO film on thesubstrate at low temperature firstly. On the other hand, since the groupIII element represented by Ga and Al has low vapor pressure and thewettability on sapphire is effective in the high temperature region, thecrystal grows at high temperature without via a buffer layer.

Since diffusion of the materials on the surface of the substrate becomesdifficult to occur and the growth film became an assembly of therod-shaped or core-shaped crystal when growing up at low temperature,the high quality crystal growth of the semiconductor level was difficultalthough it is an orienting film.

Therefore, a method of growing up via a buffer layer is usually used forthe epitaxial growth on the heterogeneous substrate. In particular, inthe case of the GaN on the sapphire substrate, it has succeeded inobtaining the GaN crystal in which the crystallinity is effective andthe surface flatness is excellent by forming the buffer layer comprisingAlN or GaN at low temperature, and then growing up at high temperature(for example, refer to Patent Literature 1 and Non Patent Literature 2).

The technology of forming the buffer layer is proposed in the case ofthe ZnO based semiconductor crystal as well as the case of the GaN basedsemiconductor crystal (for example, refer to Patent Literature 2 and NonPatent Literature 3). According to the above-mentioned method, aftergrowing up the low-temperature grown ZnO single crystal layer of about10 nm to about 100 nm in thickness at the temperature lower than 600degrees C. and then performing planarization processing by annealing,the ZnO growth is performed at the temperature lower than 800 degrees C.However, in the above-mentioned method, it is premised on the MBEtechnique and the growth temperature of high temperature growth islimited at about 800 degrees C. Moreover, according to a method of usinga Zn single material or an organic metal material as a Zn materialsource, there was a problem that raw material efficiency decreasedsharply in accordance that the high temperature growth is performed,because of the high vapor pressure of Zn.

When laminating the ZnO semiconductor film for a light emitting deviceand a light-detecting device unlike a transparent conductive film, afilm of high quality with fewer defects is required. In the case of theGaN based semiconductor layer, although the device produced by thehetero epitaxial growth on the sapphire substrate is mass-produced, thedislocation density of its film is as much as not less than 10⁸ cm⁻²grade, and it is a level of dislocation density which cannot expect theusual device operation in any semiconductor layers except the GaN based,in particular an InGaN film.

The ZnO substrate produced by the hydrothermal synthesis method and thechemical vapor transport method are available, and in order to grow upthe ZnO based semiconductor layer with fewer crystal defects, it ispreferable to homo epitaxially grow by using the ZnO substrate. Sincenot only the lattice constant of growth film and the ZnO substrate arematched, but also the coefficient of thermal expansion of the ZnOsubstrate and the growth film is the same, the homo epitaxial crystalgrown method is a method very excellent as film formation of thesemiconductor layer with fewer crystal defects.

In recent years, although the ZnO substrate produced with thehydrothermal synthesis method has a problem in respect of an impuritycontrol, the crystallinity measured from X-ray diffraction is highenough also as a semiconductor use, and since the ZnO film which growshomo epitaxial crystal layer performing lattice matching on the ZnOsubstrate may inherit the satisfactory crystallinity of the substrate,the growth of the ZnO based semiconductor layer with high internalquantum efficiency is expected.

The homo epitaxial growth on the ZnO substrate is reported with the PLDmethod, the MBE method, the MOCVD method, etc. The vapor phase epitaxialcrystal growth such as the MOCVD method also among the above-mentionedmethods is more fit for the mass production since an ultra-high vacuumis unnecessary, and the crystal growth is controllable by the gas supplyvolume which is easy to control. Therefore, the establishment of a vaporgrowth method in which the satisfactory homo epitaxial growth suitablefor the mass production also with the ZnO based semiconductor ispossible, and the development of a vapor phase epitaxial crystal growthapparatus are desired.

Although there are a lot of reports of the ZnO growth using the MOCVDmethod, there are few reports of the high temperature growth which canexpect the crystalline improvement. This is because the organic metaldecomposes near of the substrate into the single zinc substance, and thegrowth becomes difficult because of low stickiness coefficient of zinc,in the MOCVD method of the high temperature region (>800 degrees C.) ofZnO.

Moreover, the reactivity of organic metal, such as DMZn (dimethyl zinc)and DEZn (diethyl zinc), and oxygen material is high, and in growth bythe pressure of about several 100 Torr adopted by usual MOCVD for groupIII-V semiconductor, the oxygen material reacts to the organic metaleasily in the vapor phase before gas reaches a substrate (prematurereaction). As a result, it becomes a cause of a jam of a source outletunit and particle of a materials outlet unit.

There is a report that the high temperature growth is performed usingthe MOCVD method (for example, refer to Non Patent Literature 4).

In Non Patent Literature 4, it is growing homo epitaxially on the ZnOsubstrate using DMZn and O₂ gas. There is a report that the surfacewhere an atomic step appears by combining a high VI/II ratio and thesubstrate temperature of 1000 degrees C. is obtained. According to theresult of an image of the AFM (Atomic Force Microscope) in Non PatentLiterature 4, the direction of step is not match such as the ZnOsubstrate. Furthermore, a result that the height of step is also highercompared with a monolayer step is obtained. Moreover, an abnormalitypart of hexagonal prism shape with the size of about 50 nm appears, andfurther improvement is required for the homo epitaxial growth of highquality, as described in Non Patent Literature 2.

Moreover, according to the present inventors' experience, in the MOCVDmethod, the growth rate reduces as the high temperature growth isperformed, and if the material partial pressure is increased in order toavoid the reduction of the growth rate, the material utilizationefficiency is reduced by the above-mentioned premature reactions. As aresult, there is a problem that realistic material utilizationefficiency is not obtained.

In particular, the matter for which the ZnO is applied as p type becomesa great barrier of ZnO device development, and many organizations areconcentrating on applying ZnO as p type even currently. As for the ptype doping materials to the ZnO based semiconductor, a method ofdisplacing an oxygen atom to a group V element is examined by manyorganizations, and N (nitrogen), As (arsenic), P (phosphorus), Sb(antimony), etc. are mentioned to a candidate. Also among thiscandidate, the ion radius of N is the same extent as oxygen, and N isleading as the p type dopant candidate of ZnO.

It is known that many acceptors in a wide gap semiconductor are trappedin deep levels usually, and an activation rate is low near roomtemperature. For example, the case of gallium nitride having a bandgapof the same extent as ZnO, the activation rate in the room temperatureof Mg which is a p type dopant is as low as several percent, and inorder to achieve carrier concentration (the concentration exceeding1×10¹⁷ cm⁻³ is needed) used with optical devices, Mg more than 10¹⁹ cm⁻³grade is doped usually.

It has been considered conventional that high doping of the nitrogen toZnO is difficult in a high temperature region (for example, refer toPatent Literature 3).

For example, in Patent Literature 3, it is proposed the method of:forming a ZnO layer doped with high-concentration N at the lowtemperature which is about 300 degrees C. which nitrogen can dopemostly; and repeating the sequence which anneals at high temperature ofabout 800 degrees C. and forms a low concentration N doping layer. InPatent Literature 3, although the PLD method for annealing the substrateby laser is adopted, and it is a method in which rapid temperatureincreasing and rapid temperature decreasing are possible in a short timeof several minutes, the step for growing ZnO is included also in thetemperature increasing and temperature decreasing of a sample, and inparticular, therefore it is difficult to control temperature withsufficient reproducibility during the temperature decreasing which isself-cooling.

As mentioned above, although the doping efficiency of nitrogen isstrongly dependent on the growth temperature, since the crystallinityreduces and nitrogen is not activated if the substrate temperature isreduced, it is very difficult to form the p type ZnO.

Moreover, although there is also a MBE apparatus in the high vacuumprocess apparatus for performing nitrogen doping besides theabove-mentioned PLD apparatus, N plasma using a radical cell for Ndoping source is used in many cases. Thus, in the apparatus using theradical cell, if plasma power is raised in order that a radical elementis increased, there is a fault that sputtering of the internal wall ofthe cell is performed and an internal wall material is doped in ZnO.

If the internal wall material is doped in ZnO, it will also become apollution source in many cases. As a result, there was a problem that itis not only difficult to obtain desired composition and p type doping,but also the controllability of ion concentration is difficult byintroducing an impurity which is not aimed.

On the other hand, in the MOCVD method, there is a problem that the rateof the zinc which can be contributed to growth of the ZnO basedsemiconductor on the growth substrate is small, and the efficiency ofmaterial including zinc is low, according to the problem of zinc vaporpressure.

Moreover, since carbon mixes into the ZnO based semiconductor by thehydrocarbon group occurred when organic metal material gas includingzinc is decomposed, there is a problem that growth of the ZnO basedsemiconductor which does not include carbon is difficult.

CITATION LIST

-   Patent Literature 1: Japanese Patent No. 3257344-   Patent Literature 2: Japanese Patent No. 3424814-   Patent Literature 3: Japanese Patent Application Laying-Open    Publication No. 2005-223219-   Non Patent Literature 1: N. Takahashi, et al. “Atmospheric pressure    vapor-phase growth of ZnO using chloride source”, Journal of Crystal    Growth. 209 (2000), 822-   Non Patent Literature 2: H. Amano, N. Sawaki and I. Akasaki,    “Metalorganic vapor phase epitaxy growth of a high quality GaN film    using an AlN buffer layer”, Appl. Phys. Lett. 48(5), 3 February    1986, 353-   Non Patent Literature 3: H. KATO, M. SANO, K. MIYAMOTO, and T. YAO,    “Effect of O/Zn Flux Ratio on Crystalline Quality of ZnO Films Grown    by Plasma-Assisted Molecular Beam Epitaxy”, Jpn. J. Appl. Phys. Vol.    42 (2003), 2241-   Non Patent Literature 4: S. Heinze, et al. “Homo epitaxial growth of    ZnO by metalorganic vapor phase epitaxy in two-dimensional growth    mode”, Jounal of Crystal Growth 308 (2007) 170

SUMMARY OF THE INVENTION Technical Problem

In the case of ZnO based semiconductor crystal growth on theheterogeneous substrate, such as a sapphire substrate, even when thehalogenide of Zn is applied as materials, the wettability of ZnO towardthe heterogeneous substrate is low when high temperature growth isperformed, a crystalline nucleus grows sparsely without the ZnO on theheterogeneous substrate becoming a film, and it cannot become continuousmembrane easily.

Moreover, in the case where the high temperature growth is performed bythe MOCVD method or the MBE method, since growth of ZnO is difficult inhigh temperature growth even when the buffer layer is used as mentionedabove, and raw material efficiency not only worsens, but oxygen materialreacts to Zn easily when the amount of supply of Zn material isincreased, the premature reaction in the vapor phase occurs and itbecomes a cause of a particles generation.

The purpose of the present invention is to provide a hetero epitaxialgrowth method that it can grow up a ZnO based semiconductor crystal on aheterogeneous substrate, such as a sapphire substrate, at a temperaturehigher than 800 degrees C.

Moreover, the purpose of the present invention is to provide a heteroepitaxial crystal structure and a semiconductor device which are formedby using the above-mentioned hetero epitaxial growth method.

Furthermore, the purpose of the present invention is to provide a heteroepitaxial crystal growth apparatus in order to grow up a ZnO basedsemiconductor crystal on a heterogeneous substrate, such as a sapphiresubstrate, at a temperature higher than 800 degrees C.

High temperature growth is indispensable on the crystal quality of thesemiconductor layer which grows. On the other hand, if the melting pointof 1975 degrees C. of ZnO is taken into consideration, growth of about1000 degrees C. is desired as well as GaN.

The present inventors found out that a ZnO based semiconductor layer ofsatisfactory crystal quality could be grown up by supplying halogenideof zinc and/or magnesium and oxygen containing material on the ZnOsubstrate set as not less than 1000 degrees C. Moreover, the presentinventors found out that a clear atomic step appears on the surface of agrowth film by setting the material partial pressure of the halogenideof zinc and/or magnesium as not more than 1×10⁻⁴ atm (preferably notmore than 3×10⁻⁵ atm).

The purpose of the present invention is to provide a homo epitaxialgrowth method which can grow the ZnO based semiconductor crystal on theZnO substrate at a temperature higher than 1000 degrees C.

Moreover, the purpose of the present invention is to provide a homoepitaxial crystal structure and a semiconductor device which are formedby using the above-mentioned homo epitaxial growth method.

Furthermore, the purpose of the present invention is to provide a homoepitaxial crystal growth apparatus in order to grow up the ZnO basedsemiconductor crystal on the ZnO substrate at a temperature higher than1000 degrees C.

On the other hand, in the HVPE method, since it is growing up inatmospheric pressure or the state near atmospheric pressure, plasma usedby the MBE method or the PLD method cannot be used for applying thenitrogen doping of a group V element as a p type impurity by the HVPEmethod. Moreover, if it uses in the state of N₂ gas, the triple bond ofN is strong and it is not function as nitrogen doping gas in the growthtemperature which is degree 1000 degrees C. On the other hand, as anitrogen source, the gas which can become a nitrogen source exists evenif it is plasma-less gas, such as N₂O and NO₂ gas.

However, in the HVPE method, the premature reaction in which a reactionoccurs before material gas reaches to the substrate is prevented bygrowing up by the small system of an equilibrium constant using H₂O gasand a halogenide. Then, introducing the gas having oxidizing power, suchas N₂O and NO₂ gas, in its state causes the premature reaction, and itmay cause occurring of particle and deterioration of raw materialefficiency.

On the other hand, As, P, and Sb which are other group V elements are asolid at room temperature, and are not fit for doping of the vapor phaseepitaxial crystal growth applying the single substance as the startingmaterial.

The present inventors found out that enough nitrogen can be doped foreven if the growth temperature is set up highly by reacting p typeimpurity material gas as hydride gas of group V material to the halidegas including Zn. Furthermore, it has verified that the crystallinity isimproved when ZnO doped with the p type impurity is grown up by theabove-mentioned HVPE method, even if it is a growth temperature regionto which the crystallinity reduces by undoped ZnO etc. conventionally.

The purpose of the present invention is to provide a ZnO basedsemiconductor which can prevent mixing of the impurity which is notaimed and can dope a p type impurity enough also at high temperature,and a fabrication method and a fabricating apparatus for the ZnO basedsemiconductor.

Solution to Problem

According to an aspect of the invention, a hetero epitaxial growthmethod comprises forming a buffer layer on a heterogeneous substrate;and performing crystal growth of a zinc oxide based semiconductor layeron the buffer layer using a halogenated group II metal and an oxygenmaterial.

According to another aspect of the invention, a hetero epitaxial crystalstructure comprises a heterogeneous substrate; a buffer layer disposedon the heterogeneous substrate; and a zinc oxide based semiconductorlayer disposed on the buffer layer and formed by crystal growth using ahalogenated group II metal and an oxygen material.

According to another aspect of the invention, a semiconductor devicecomprises a heterogeneous substrate; a buffer layer disposed on theheterogeneous substrate; a n type zinc oxide based semiconductor layerdisposed on the buffer layer and impurity-doped with a n type impurity;a zinc oxide based semiconductor active layer disposed on the n typezinc oxide based semiconductor layer; and a p type zinc oxide basedsemiconductor layer disposed on the active layer and impurity-doped witha p type impurity.

According to another aspect of the invention, a hetero epitaxial growthapparatus comprises a first source zone for holding a first group IImetallic material including a zinc single metal substance; a secondsource zone for holding a second group II metallic material including agroup II single metal substance except zinc; a first halogen gassupplying unit for supplying first halogen gas to the first source zone;a second halogen gas supplying unit for supplying second halogen gas tothe second source zone; an oxygen material supplying unit for supplyingan oxygen material including oxygen; a first doping gas supplying unitfor supplying n type doping gas for impurity-doping a n type impurity; asecond doping gas supplying unit for supplying p type doping gas forimpurity-doping a p type impurity; and a growth zone for reacting thehalogenated gas of the first and second group II metal generated fromthe group II metallic material, the oxygen material, the n type and ptype doping gas.

According to another aspect of the invention, a homo epitaxial growthmethod comprises introducing reactant gas mixing zinc containing gas andoxygen containing gas on a zinc oxide substrate; and performing crystalgrowth of a zinc oxide based semiconductor layer on the zinc oxidesubstrate.

According to another aspect of the invention, a homo epitaxial crystalstructure comprises a zinc oxide substrate; and a zinc oxide basedsemiconductor layer disposed on the zinc oxide substrate and formed bycrystal growth using a halogenated group II metal and an oxygenmaterial.

According to another aspect of the invention, a semiconductor devicecomprises a zinc oxide substrate; a n type zinc oxide basedsemiconductor layer disposed on the zinc oxide substrate andimpurity-doped with an type impurity; a zinc oxide based semiconductoractive layer disposed on the n type zinc oxide based semiconductorlayer; and a p type zinc oxide based semiconductor layer disposed on theactive layer and impurity-doped with a p type impurity.

According to another aspect of the invention, a homo epitaxial growthapparatus comprises a first source zone for holding a first group IImetallic material including a zinc single metal substance; a secondsource zone for holding a second group II metallic material including agroup II single metal substance except zinc; a first halogen gassupplying unit for supplying first halogen gas to the first source zone;a second halogen gas supplying unit for supplying second halogen gas tothe second source zone; an oxygen material supplying unit for supplyingan oxygen material including oxygen; a first doping gas supplying unitfor supplying n type doping gas for impurity-doping a n type impurity; asecond doping gas supplying unit for supplying p type doping gas forimpurity-doping a p type impurity; and a growth zone for reacting thehalogenated gas of the first and second group II metal generated fromthe group II metallic material, the oxygen material, the n type and ptype doping gas.

According to another aspect of the invention, a fabrication method for aZnO based semiconductor comprises introducing reactant gas mixinghalogenated group II metallic gas including zinc and oxygen containinggas on one of a substrate and a semiconductor layer; and introducinghydride gas of group V as p type impurity material gas on one of thesubstrate and the semiconductor layer, wherein crystal growth of thezinc oxide based semiconductor layer doped with a p type impurity isperformed on one of the substrate and the semiconductor layer.

According to another aspect of the invention, an apparatus forfabricating a ZnO based semiconductor comprises a source zone forholding a group II metallic material including a zinc single metalsubstance; a halogen gas supplying unit for supplying halogen gas to thesource zone; an oxygen material supplying unit for supplying an oxygenmaterial including oxygen; a hydride gas supplying unit for supplyinghydride gas of group V for impurity-doping a p type impurity; and agrowth zone for reacting the hydride gas of the group V, the halogenatedgroup II metallic gas generated from the group II metallic material, andthe oxygen material, wherein a gas supply line connected to the hydridegas supplying unit is disposed by extending to the growth zone, and agas supplying outlet port of the aforesaid gas supply line is disposedthe upper side of the substrate or the semiconductor.

Advantageous Effects of Invention

According to the present invention, the hetero epitaxial growth methodthat the ZnO based semiconductor crystal can be grown up on theheterogeneous substrate, such as a sapphire substrate, at a temperaturehigher than 800 degrees C. can be provided.

Moreover, according to the present invention, the hetero epitaxialcrystal structure and the semiconductor device which are formed by usingthe above-mentioned hetero epitaxial growth method can be provided.

Moreover, according to the present invention, the hetero epitaxialcrystal growth apparatus in order to grow up the ZnO based semiconductorcrystal on the heterogeneous substrate, such as a sapphire substrate, ata temperature higher than 800 degrees C. can be provided.

According to the present invention, the homo epitaxial growth methodwhich can grow up the ZnO based semiconductor crystal on the ZnOsubstrate at a temperature higher than 1000 degrees C. can be provided.

Moreover, according to the present invention, the homo epitaxial crystalstructure and the semiconductor device which are formed by using theabove-mentioned homo epitaxial growth method can be provided.

Moreover, according to the present invention, the homo epitaxial crystalgrowth apparatus in order to grow up the ZnO based semiconductor crystalon the ZnO substrate at a temperature higher than 1000 degrees C. can beprovided.

According to the present invention, since the step of introducing thehydride gas of group V material as p type impurity material gas inaddition to the step of introducing the reactant gas which mixed thehalogenated group II metallic gas which includes zinc at least andoxygen containing gas, on the substrate or the semiconductor layer isprovided, the premature reaction can be prevented and the p typeimpurity can fully be doped also at high growth temperature. Moreover,the crystal quality of the ZnO based semiconductor can also be improvedby applying high growth temperature. Moreover, since plasma is not used,mixing of the impurity which is not aimed can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows a hetero epitaxial growth method of a hetero epitaxialcrystal structure according to a first embodiment of the presentinvention, and is a schematic cross-sectional configuration diagramshowing a buffer layer formation process.

FIG. 1B shows the hetero epitaxial growth method of the hetero epitaxialcrystal structure according to the first embodiment of the presentinvention, and is a schematic cross-sectional configuration diagramshowing a formation process of the ZnO facet by halogenide vapor phaseepitaxy.

FIG. 1C shows the hetero epitaxial growth method of the hetero epitaxialcrystal structure according to the first embodiment of the presentinvention, and is a schematic cross-sectional configuration diagramshowing a formation process of the ZnO layer by halogenide vapor phaseepitaxy.

FIG. 2 is a schematic bird's-eye view structural drawing of a heteroepitaxial crystal structure in the formation process of the ZnO layer byhalogenide vapor phase epitaxy, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 3A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method according to the first embodiment of thepresent invention, and is an optical microscope photograph of thesurface of the ZnO layer.

FIG. 3B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method according to the first embodiment of thepresent invention, and is a bird's-eye view SEM photograph of the ZnOlayer.

FIG. 4A shows a hetero epitaxial crystal structure formed by a heteroepitaxial growth method according to a comparative example of thepresent invention, and is an optical microscope photograph on thesurface of the ZnO layer.

FIG. 4B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method according to the comparative example ofthe present invention, and is a bird's-eye view SEM photograph of theZnO layer.

FIG. 5A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of VI/II ratio=200) according tothe first embodiment of the present invention, and is an opticalmicroscope photograph on the surface of the ZnO layer.

FIG. 5B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of VI/II ratio=200) according tothe first embodiment of the present invention, and is a bird's-eye viewSEM photograph of the ZnO layer.

FIG. 6A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 20 minutes of growth period)according to the first embodiment of the present invention, and is anoptical microscope photograph on the surface of the ZnO layer.

FIG. 6B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 20 minutes of growth period)according to the first embodiment of the present invention, and is abird's-eye view SEM photograph of the ZnO layer.

FIG. 7A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 60 minutes of growth period)according to the first embodiment of the present invention, and is anoptical microscope photograph on the surface of the ZnO layer.

FIG. 7B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 60 minutes of growth period)according to the first embodiment of the present invention, and is abird's-eye view SEM photograph of the ZnO layer.

FIG. 8A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 180 minutes of growth period)according to the first embodiment of the present invention, and is anoptical microscope photograph on the surface of the ZnO layer.

FIG. 8B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 180 minutes of growth period)according to the first embodiment of the present invention, and is abird's-eye view SEM photograph of the ZnO layer.

FIG. 9A shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 360 minutes of growth period)according to the first embodiment of the present invention, and is anoptical microscope photograph on the surface of the ZnO layer.

FIG. 9B shows the hetero epitaxial crystal structure formed by thehetero epitaxial growth method (example of 360 minutes of growth period)according to the first embodiment of the present invention, and is abird's-eye view SEM photograph of the ZnO layer.

FIG. 10 shows the X-ray rocking curve of the ZnO crystal of the heteroepitaxial crystal structure formed by the hetero epitaxial growth method(example of ZnO (002) plane) according to the first embodiment of thepresent invention.

FIG. 11 shows the X-ray rocking curve of the ZnO crystal of the heteroepitaxial crystal structure formed by the hetero epitaxial growth method(example of ZnO (101) plane) according to the first embodiment of thepresent invention.

FIG. 12 is a drawing showing the relation between full width at halfmaximum FWHM (arc sec.) and crystal growth period (min.) of the X-rayrocking curve of the ZnO crystal of the hetero epitaxial crystalstructure formed by the hetero epitaxial growth method according to thefirst embodiment of the present invention (example of ZnO (002) plane,and example of ZnO (101) plane).

FIG. 13 is a schematic diagram of an epitaxial crystal growth apparatusapplying to the hetero epitaxial growth method according to the firstembodiment of the present invention.

FIG. 14A shows an optical microscope photograph of the surface of a ZnObuffer layer formed by reacting a zinc metal to water, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 14B shows an optical microscope photograph of the surface of a ZnObuffer layer formed by reacting zinc chloride to water, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 15A shows a bird's-eye view SEM photograph of the surface of theZnO buffer layer which formed by reacting the zinc metal to the water,in the hetero epitaxial growth method according to the first embodimentof the present invention (Step 1).

FIG. 15B shows a bird's-eye view SEM photograph of the surface of theZnO buffer layer annealed with nitrogen gas in a steam atmosphere, inthe hetero epitaxial growth method related to the first embodiment ofthe present invention (Step 2).

FIG. 15C shows a bird's-eye view SEM photograph of the surface of theZnO layer formed by reacting the zinc chloride and the watercontinuously, in the hetero epitaxial growth method according to thefirst embodiment of the present invention (Step 3).

FIG. 16 shows an example of a result of a measurement of the X-raydiffraction measurement (2 theta-omega method) for explaining anorientation of a crystal of the ZnO layer corresponding to each step ofFIG. 15, in the hetero epitaxial growth method according to the firstembodiment of the present invention.

FIG. 17A shows the X-ray rocking curve for explaining the crystallinityof the ZnO layer corresponding to each step of FIG. 15 (example of ZnO(002) plane), in the hetero epitaxial growth method according to thefirst embodiment of the present invention.

FIG. 17B shows the X-ray rocking curve for explaining the crystallinityof the ZnO layer corresponding to each step of FIG. 15 (example of ZnO(101) plane), in the hetero epitaxial growth method according to thefirst embodiment of the present invention.

FIG. 18 is a drawing showing the relation between the growth temperatureand growth period in order to investigate the thickness dependency of abuffer layer, in the hetero epitaxial growth method according to thefirst embodiment of the present invention.

FIG. 19A shows an optical microscope photograph in the case of settingthe growth period of the buffer layer to 0 minute, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 19B shows a bird's-eye view SEM photograph corresponding to FIG.19A.

FIG. 19C shows an optical microscope photograph in the case of settingthe growth period of the buffer layer to 10 minutes, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 19D shows a bird's-eye view SEM photograph corresponding to FIG.19C.

FIG. 19E shows an optical microscope photograph in the case of settingthe growth period of the buffer layer to 30 minutes, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 19F shows a bird's-eye view SEM photograph corresponding to FIG.19E.

FIG. 19G shows an optical microscope photograph in the case of settingthe growth period of the buffer layer to 60 minutes, in the heteroepitaxial growth method according to the first embodiment of the presentinvention.

FIG. 19H shows a bird's-eye view SEM photograph corresponding to FIG.19G.

FIG. 20A shows a bird's-eye view SEM photograph in the case of settinggrowth period of the buffer layer to 20 minutes and setting growthperiod of the ZnO layer to 20 minutes, on an a-plane sapphire substrate,in the hetero epitaxial growth method according to the first embodimentof the present invention.

FIG. 20B shows a bird's-eye view SEM photograph at a high magnifictioncorresponding to FIG. 20A.

FIG. 20C shows a bird's-eye view SEM photograph in the case of settinggrowth period of the buffer layer to 60 minutes and setting growthperiod of the ZnO layer to 20 minutes, on the a-plane sapphiresubstrate, in the hetero epitaxial growth method according to the firstembodiment of the present invention.

FIG. 20D shows a expansion bird's-eye view SEM photograph at a highmagnifiction corresponding to FIG. 20C.

FIG. 21 is an example of the result of a measurement of the X-raydiffraction measurement (2 theta-omega method) after performing crystalgrowth of the ZnO layer by the HVPE method during 60 minutes afterperforming formation of the buffer layer during 10 minutes, on thea-plane sapphire substrate, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 22 is an example of the result of a measurement of the X-raydiffraction measurement (2 theta-omega method) after performing crystalgrowth of the ZnO layer by the HVPE method during 60 minutes afterperforming formation of the buffer layer during 30 minutes, on thea-plane sapphire substrate, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 23 is an example of the result of a measurement of the X-raydiffraction measurement (2 theta-omega method) after performing crystalgrowth of the ZnO layer by the HVPE method during 60 minutes afterperforming formation of the buffer layer during 60 minutes, on thea-plane sapphire substrate, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 24A shows an example of the X-ray rocking curve after performingcrystal growth of the ZnO layer by the HVPE method during 60 minutesafter performing formation of the buffer layer during 30 minutes and 60minutes, respectively, on the a-plane sapphire substrate, in the heteroepitaxial growth method according to the first embodiment of the presentinvention (ZnO (002) plane).

FIG. 24B shows an example of the X-ray rocking curve in the case of theZnO (101) plane which grows the ZnO layer on the same conditions, in thehetero epitaxial growth method according to the first embodiment of thepresent invention.

FIG. 25 is an explanatory diagram of a X-ray phi scan of the ZnO layerformed on the a-plane sapphire substrate, in the hetero epitaxial growthmethod according to the first embodiment of the present invention.

FIG. 26 shows a result of a measurement of the X-ray phi scan of the ZnO(101) plane based on the existence or nonexistence of the buffer layerformed on the a-plane sapphire substrate, in the hetero epitaxial growthmethod according to the first embodiment of the present invention.

FIG. 27A is a schematic cross-sectional configuration diagram of the ZnOlayer performed the crystal growth on a r-plane sapphire substratewithout forming the buffer layer, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 27B shows a surface bird's-eye view SEM photograph after performingcrystal growth of the ZnO layer by the HVPE method during 60 minutes onthe r-plane sapphire substrate without forming the buffer layer, in thehetero epitaxial growth method according to the first embodiment of thepresent invention.

FIG. 28A is a schematic cross-sectional configuration diagram of the ZnOlayer performed the crystal growth after forming the buffer layer on ther-plane sapphire substrate, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 28B shows a surface bird's-eye view SEM photograph after performingthe crystal growth of the ZnO layer by the HVPE method during 60 minutesafter forming the buffer layer on the r-plane sapphire substrate, in thehetero epitaxial growth method according to the first embodiment of thepresent invention.

FIG. 29A shows a surface bird's-eye view SEM photograph performed thecrystal growth of the buffer layer at 400 degrees C. during 60 minuteson the r-plane sapphire substrate, in the hetero epitaxial growth methodaccording to the first embodiment of the present invention.

FIG. 29B shows a surface bird's-eye view SEM photograph of the ZnO layerperformed the crystal growth further at 1000 degrees C. during 60minutes, in the hetero epitaxial growth method according to the firstembodiment of the present invention.

FIG. 30 shows an example of a result of a measurement of X-raydiffraction measurement (2 theta-omega method) of the ZnO layer formedas shown in FIG. 29.

FIG. 31 is a schematic cross-sectional configuration diagram of asemiconductor device having the hetero epitaxial crystal structureformed by the hetero epitaxial growth method according to the firstembodiment of the present invention.

FIG. 32A shows a homo epitaxial growth method of a homo epitaxialcrystal structure according to a second embodiment of the presentinvention, and is a schematic cross-sectional configuration diagramshowing a process for which a ZnO substrate is prepared.

FIG. 32B shows the homo epitaxial growth method of the homo epitaxialcrystal structure according to the second embodiment of the presentinvention, and is a schematic cross-sectional configuration diagramshowing a formation process of the ZnO based semiconductor layer byhalogenide vapor phase epitaxy.

FIG. 33A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=700 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 33B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=700 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 34A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=800 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 34B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=800 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 35A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=900 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 35B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=900 degreesC.), and is an AFM photograph of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 36A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=1000 degreesC.) according to the second embodiment of the present invention, and isan AFM photograph of the surface of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 36B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (crystal growth temperature T_(g)=1000 degreesC.) according to the second embodiment of the present invention, and isan AFM photograph of the surface of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 37A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=20) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 2 μm squarepart.

FIG. 37B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=20) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 20 μm squarepart.

FIG. 38A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=50) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 2 μm squarepart.

FIG. 38B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=50) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 20 μm squarepart.

FIG. 39A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=1000) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 2 μm squarepart.

FIG. 39B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., and VI/II ratio=1000) according to the secondembodiment of the present invention, and is an AFM photograph of thesurface of the ZnO based semiconductor layer surface of 20 μm squarepart.

FIG. 40A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=1 hour)according to the second embodiment of the present invention, and is anAFM photograph of the surface of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 40B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=1 hour)according to the second embodiment of the present invention, and is anAFM photograph of the surface of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 41A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=2hours) according to the second embodiment of the present invention, andis an AFM photograph of the surface of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 41B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=2hours) according to the second embodiment of the present invention, andis an AFM photograph of the surface of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 42A shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=6hours) according to the second embodiment of the present invention, andis an AFM photograph of the surface of the ZnO based semiconductor layersurface of 2 μm square part.

FIG. 42B shows the homo epitaxial crystal structure formed by the homoepitaxial growth method (an example of crystal growth temperatureT_(g)=1000 degrees C., VI/II ratio=20, and crystal growth period=6hours) according to the second embodiment of the present invention, andis an AFM photograph of the surface of the ZnO based semiconductor layersurface of 20 μm square part.

FIG. 43 shows an AFM photograph showing the surface morphology of 1 MLstep of +c-plane ZnO based semiconductor layer surface in alignment withm-axial direction <1-10> and a-axial direction <110>, in the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention.

FIG. 44A is an enlarged drawing of an AFM photograph showing the surfacemorphology of 1 ML step of +c-plane ZnO based semiconductor layersurface in alignment with m-axial direction <1-10> and a-axial direction<110>, in the homo epitaxial crystal structure formed by the homoepitaxial growth method according to the second embodiment of thepresent invention.

FIG. 44B is a schematic planar structure diagram of the step morphologyof 1 ML step corresponding to FIG. 44A.

FIG. 44C is a schematic cross-sectional configuration diagram of thestep morphology between A-B corresponding to FIG. 44B.

FIG. 44D is an explanatory diagram of the m-axial direction <1-10> andthe a-axial direction <110> in hexagonal shape.

FIG. 45 shows the X-ray rocking curve of the ZnO crystal of the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention (example ofZnO (002) plane).

FIG. 46 shows the X-ray rocking curve of the ZnO crystal of the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention (example ofZnO (101) plane).

FIG. 47 shows a SIMS analysis result of the ZnO crystal of the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention (polarity ofdetected ion: −).

FIG. 48 shows a SIMS analysis result of the ZnO crystal of the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention (polarity ofdetected ion: +).

FIG. 49 shows a SIMS analysis result showing the depth directiondependence of the chlorine concentration of the ZnO crystal of the homoepitaxial crystal structure formed by the homo epitaxial growth methodaccording to the second embodiment of the present invention.

FIG. 50 shows a result of a measurement of the electricalcharacteristics of the ZnO film of the homo epitaxial crystal structureformed by the homo epitaxial growth method according to the secondembodiment of the present invention, and is C-V characteristics of theMOS structure.

FIG. 51 is a schematic cross-sectional configuration diagram of thesample applying to C-V measurement of FIG. 50.

FIG. 52 is a relationship diagram of the minimum capacitance C_(min)(F/cm²) and the maximum depletion layer width W_(max) (μm) toward donorconcentration N_(D) (cm⁻³).

FIG. 53 is a schematic cross-sectional configuration diagram of asemiconductor device having the homo epitaxial crystal structure formedby the homo epitaxial growth method according to the second embodimentof the present invention.

FIG. 54 is a drawing showing an AFM image of 20 μm square of an undopedZnO based semiconductor layer surface formed by the HVPE method.

FIG. 55 is a drawing showing an AFM image of 20 μm square of a nitrogendoped ZnO based semiconductor layer surface formed by a fabricationmethod for a ZnO based semiconductor according to a third embodiment ofthe present invention.

FIG. 56A is a drawing showing the relation of crystal growth drivingforce and growth temperature by the thermodynamic analysis in the caseof changing the VI/II ratio.

FIG. 56B is a drawing showing the comparison with the case where thereis no supply of ammonia gas, in the relation of the crystal growthdriving force and the growth temperature by the thermodynamic analysisin the case of changing the VI/II ratio.

FIG. 57 is a drawing showing the relation between the growth rate andthe growth temperature of a nitrogen doped ZnO layer for every ammoniagas partial pressure.

FIG. 58 is a drawing which classifies whether the nitrogen doped ZnOlayer is growing or etching, based on the data of FIG. 57.

FIG. 59 is a drawing showing that the nitrogen concentration of thenitrogen doped ZnO changes in accordance with growth temperature.

FIG. 60 is a drawing showing a secondary ion mass analytical measurementresult in the representative case of the nitrogen doped ZnO layerfabricated by the fabrication method of the ZnO based semiconductor ofthe present invention.

FIG. 61 is a drawing showing a schematic configuration example of afabricating apparatus applying the fabrication method of the ZnO basedsemiconductor according to the third embodiment of the presentinvention.

FIG. 62 is a drawing showing a configuration example applying a p typeZnO based semiconductor layer as a multilayer film in a ZnO basedsemiconductor element fabricated by the fabrication method of the ZnObased semiconductor according to the third embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Next, embodiments of the invention will be described with reference todrawings. It explains simple by attaching the same reference numeral asthe same block or element to below, and avoiding duplication ofdescription. Drawings are schematic, not actual, and may be inconsistentin between in scale, ratio, etc.

The embodiment shown in the following exemplifies the apparatus andmethod for materializing the technical idea of this invention, and theembodiments of the invention do not specify assignment of each componentparts, etc. as the following. Various changes can be added to thetechnical idea of the present invention in scope of claims.

The “ZnO based compound semiconductor” as used in the followingexplanation means the group II oxide semiconductor containing the groupII element Zn.

First Embodiment Hetero Epitaxial Growth Method

A hetero epitaxial growth method of a hetero epitaxial crystal structureaccording to a first embodiment of the present invention will beexplained with reference to FIG. 1.

A buffer layer formation process is expressed as shown in FIG. 1A. Aformation process of a ZnO facet 44 by halogenide vapor phase epitaxy isexpressed as shown in FIG. 1B. A formation process of a ZnO layer byhalogenide vapor phase epitaxy is expressed as shown in FIG. 1C.

(a) First of all, as shown in FIG. 1A, a ZnO template is formed by alaser ablation method on a heterogeneous substrate 40, using an a-planesapphire substrate for example as the heterogeneous substrate 40. TheZnO template is composed of a buffer layer 42. The buffer layer 42 canalso be formed with the orienting film of an oxide, or the orientingfilm of nitride. More specifically, for example, the orienting film ofthe oxide composed of ZnO or MgO, or the orienting film of the nitridecomposed of AlN or GaN is applicable. The temperature for forming theZnO template is about 500 degrees C., for example. Moreover, thethickness of the buffer layer 42 is about 0.3 μm, for example.

As other formation methods of the ZnO template, a sputtering method, apulse laser method, a MBE method, a HVPE method, etc. can be used, forexample. Alternatively, a VPE method, etc. for merely reacting steam ofZn to water vapor are also applicable. In addition, in these formationmethods, not less than 400 degrees C. of growth temperature ispreferable, for example. It is because the performance of an underlyingcrystal substrate can be reflected to the ZnO template the moresatisfactory, the more it becomes high temperature. The VPE method formerely reacting steam of Zn to vapor is later described with referenceto FIG. 14 to FIG. 30.

In particular, considering compatibility with the subsequent hightemperature growth using a halogenated group II metal, the HVPE method,the VPE method, etc. achievable in the same growth furnace arepreferable.

Although the growth temperature of the buffer layer 42 is not specifiedin particular, the orientation of the buffer layer 42 can be improvedand the crystallinity of a subsequent high temperature grown film alsoimproves by heating the heterogeneous substrate 40 to the moderatetemperature level (to 500 degrees C.). Although a thickness of the filmis also not specified in particular, since the orientation and surfaceflatness of the buffer layer 42 may deteriorate with the method for filmdeposition of the buffer layer 42 if the film thickness is too thick,and it will not become continuous membrane if the film thickness is toothin, the film thickness of about 0.02 μm to about 0.5 μm is preferable,for example.

As for the ZnO crystal formed on the above-mentioned a-plane sapphiresubstrate, the c-plane oriented in the c-axial direction is obtained.

In addition, a silicon substrate, a SiC substrate, a GaAs substrate, aGaP substrate, a GaN based substrate, etc. are applicable as theheterogeneous substrate 40 except the above-mentioned a-plane sapphiresubstrate.

(b) Next, as shown in FIG. 1B, a ZnO facet 44 is formed on the bufferlayer 42 by the HVPE method. More specifically, the halogenide vaporphase epitaxy using zinc chloride (ZnCl₂) and water (H₂O) is applied. Asthe crystal growth conditions, the partial pressure PZnCl₂ of ZnCl₂ isset as about 2.2×10⁻⁵ atm against total pressure 1 atm, for example, theVI/II ratio which is a supply ratio between oxygen which is a group VIelement and Zn which is group II elements is set to about 20 to 200, forexample, the crystal growth temperature T_(g) is set as about 1000degrees C., for example, and the crystal growth period is set up inabout 1 hour, for example. Here, in the case of partial pressure PZnCl₂of ZnCl₂=2.2×10⁻⁵ atm, if the VI/II ratio=20 to 200, it will be set topartial pressure PH₂O of H₂O=4.4×10⁻⁴ to 4.4×10⁻³ atm.

In the hetero epitaxial growth method according to the first embodiment,the temperature T_(g) of crystal growth is enforcing the hightemperature growth method higher than 800 degrees C. If the meltingpoint of 1975 degrees C. of the ZnO crystal is taken into consideration,⅓ to about ½ of the melting point is needed, and the more thetemperature T_(g) of crystal growth is high, the more the satisfactorycrystal of quality can be obtained. Therefore, in the above-mentionedexample, the growth of about 1000 degrees C. same as GaN is performed.

(c) Next, a ZnO layer 46 of desired thickness is formed by performinglateral growth and vertical growth further for the ZnO facet 44 formedon the buffer layer 42 by using the HVPE method, setting up the crystalgrowth period still longer, as shown in FIG. 1C. More specifically, thehalogenide vapor phase epitaxy using zinc chloride (ZnCl₂), water (H₂O),and N₂ which is carrier gas is applied as well as the formation of theZnO facet 44. As the crystal growth conditions, the partial pressurePZnCl₂ of ZnCl₂ is set as about 2.2×10⁻⁵ atm, for example, the VI/IIratio which is a supply ratio between the oxygen which is a group VIelement and the Zn which is group II elements is set as about 20 toabout 200, for example, the crystal growth temperature T_(g) is set asabout 1000 degrees C., for example, and the crystal growth period is setup in about 1 hour to about 6 hours, for example.

In the hetero epitaxial growth method according to the first embodiment,it is characterized by obtaining the single crystal of ZnO by depositingthe buffer layer of ZnO at low temperature on the heterogeneoussubstrate, and then mixing Zn material composed of a halogenide of Znand oxygen material in the vapor phase with high temperature at not lessthan 800 degrees C.

The wettability of ZnO can be improved by forming the buffer layer 42 toimprove the wettability of ZnO on the heterogeneous substrate 40, suchas ZnO, before the high temperature growth using Zn material composed ofhalogenide.

(High Temperature ZnO Growth on Buffer Layer)

In the hetero epitaxial growth method according to the first embodiment,a schematic bird's-eye view structure of the hetero epitaxial crystalstructure in a formation process of the ZnO layer by the HVPE method isexpressed as shown in FIG. 2. An aspect that the lateral growth (GL) andvertical growth (GV) of the ZnO facet 44 are performed on a surface 40 aof the heterogeneous substrate 40 is shown schematically.

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, an example of theoptical microscope photograph on the surface of the ZnO layer isexpressed as shown in FIG. 3A. Moreover, a bird's-eye view SEMphotograph of the ZnO layer corresponding to FIG. 3A is expressed asshown in FIG. 3B. The example of FIG. 3A and FIG. 3B corresponds to thestructure of FIG. 1B and FIG. 2. That is, it is an example which formsthe ZnO facet 44 on the buffer layer 42 formed on the a-plane sapphiresubstrate using the a-plane sapphire substrate as the heterogeneoussubstrate 40.

As the crystal growth conditions, partial pressure P_(ZnCl2) of ZnCl₂ isset as 2.2×10⁻⁵ atm, the VI/II ratio is set as 20, the crystal growthtemperature T_(g) is set as 1000 degrees C., and the crystal growthperiod is set up in 1 hour.

Moreover, in a hetero epitaxial crystal structure formed by a heteroepitaxial growth method according to a comparative example of thepresent invention, an example of the optical microscope photograph onthe surface of the ZnO layer is expressed as shown in FIG. 4A. Moreover,a bird's-eye view SEM photograph of the ZnO layer corresponding to FIG.4A is expressed as shown in FIG. 4B. The example of FIG. 4A and FIG. 4Bcorresponds to the example which forms the ZnO crystal directly on thea-plane sapphire substrate not using the buffer layer 42 in thestructure of FIG. 1B.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set 2.2×10⁻⁵ atm, the VI/II ratio is set as 600, the crystalgrowth temperature T_(g) is set as 1000 degrees C., and the crystalgrowth period is set up in 1 hour.

By comparison of FIG. 3 and FIG. 4, when there is the buffer layer 42,the lateral growth (GL) of the ZnO facet 44 is accelerated, and thevertical growth (GV) of the ZnO facet 44 is further performed. On theother hand, when there is no buffer layer 42, the lateral growth (GL) ofthe ZnO facet 44 hardly occurs, but the vertical growth (GV) of the ZnOfacet 44 mainly occurs. Accordingly, since the growth inside the planeis not performed even if the crystallinity of the ZnO crystal itselfformed on the plane of the ZnO facet 44 is satisfactory, it is difficultto form the ZnO epitaxial growth layer. In addition, when direct growthof the ZnO is performed on the a-plane sapphire substrate on conditionof the VI/II ratio=20 same as the conditions of FIG. 3, the nucleusgrowth of ZnO is observed extremely sparsely.

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, an optical microscopephotograph on the surface of the ZnO layer in the example of VI/IIratio=20 is expressed as shown in FIG. 3A, and a SEM photograph of theZnO layer corresponding to FIG. 3A is expressed as shown in FIG. 3B.Similarly, the optical microscope photograph on the surface of a ZnOlayer in the example of VI/II ratio=200 is expressed as shown in FIG.5A, and a bird's-eye view SEM photograph of the ZnO layer correspondingto FIG. 5A is expressed as shown in FIG. 5B.

As the crystal growth conditions, the partial pressure PZnCl₂ of ZnCl₂is set as 2.2×10⁻⁵ atm, the crystal growth temperature T_(g) is set as1000 degrees C., and the crystal growth period is set up in 1 hour.

As clearly from FIG. 3A and FIG. 3B, in the case of the VI/II ratio=20,since the partial pressure PH₂O of H₂O is set as 4.4×10⁻⁴ atm, and thesurface migration of molecules of ZnCl₂ occurs easily, the lateralgrowth of the ZnO facet 44 occurs easily. On the other hand, as clearlyfrom FIG. 5A and FIG. 5B, in the case of the VI/II ratio=200, thepartial pressure PH₂O of H₂O is set to 4.4×10⁻³ atm, the more thepartial pressure PH₂O of H₂O is high, the more the surface migration ofZnCl₂ molecules is suppressed, and the result that the lateral growth(GL) does not occur easily although the vertical growth (GV) is occurredis obtained.

Therefore, when accelerating the lateral growth (GL) of the ZnO facet 44and obtaining the ZnO layer 46 of desired thickness, it proves that lowVI/II ratio growth becomes an important growing condition.

Thus, since the speed of lateral growth becomes slow when the VI/IIratio in the case of crystal growth is large, the VI/II ratio iseffective to apply not more than 100. Moreover, since the speed ofcrystal growth becomes slow when the VI/II ratio in the case of crystalgrowth is small, the VI/II ratio is effective to apply more than 1.

Moreover, it may not be limited to the above-mentioned VI/II ratio, theVI/II ratio may be set as value of more than 1 to not more than 100 (forexample, 20) until the continuous membrane is made because the facetcombines, for example, and then the VI/II ratio may be set as value ofmore than 100 (for example, 200).

(Growth Period Dependence of High Temperature ZnO)

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, an optical microscopephotograph on the surface of the ZnO layer in the case that the growthperiod is 20 minutes is expressed as shown in FIG. 6A, and a bird's-eyeview SEM photograph of the ZnO layer corresponding to FIG. 6A isexpressed as shown in FIG. 6B. Similarly, an optical microscopephotograph on the surface of the ZnO layer in the case that the growthperiod is 60 minutes is expressed as shown in FIG. 7A, a bird's-eye viewSEM photograph of the ZnO layer corresponding to FIG. 7A is expressed asshown in FIG. 7B, an optical microscope photograph on the surface of theZnO layer in the case that the growth period is 180 minutes is expressedas shown in FIG. 8A, a bird's-eye view SEM photograph of the ZnO layercorresponding to FIG. 8A is expressed as shown in FIG. 8B, an opticalmicroscope photograph on the surface of the ZnO layer in the case thatthe growth period is 360 minutes is expressed as shown in FIG. 9A, and abird's-eye view SEM photograph of the ZnO layer corresponding to FIG. 9Ais expressed as shown in FIG. 9B.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set as 2.2×10⁻⁵ (atm), the VI/II ratio is set as 20, and thecrystal growth temperature T_(g) is set as 1000 degrees C.

According to the hetero epitaxial growth method according to the firstembodiment, as for the high temperature ZnO layer, it proves an aspectthat the lateral crystal growth (GL) progresses with the passage of timeof crystal growth, the vertical crystal growth (GV) progresses further,and the epitaxial layer grows.

The high temperature growth using the halogenide can reduce thecrystalline nucleus on the buffer layer 42 as compared with the MBEmethod or the MOCVD method. Moreover, the buffer layer 42 improves thewettability of ZnO. Accordingly, the high temperature growth using thehalogenide follows the growth process which forms the film by the greatcrystalline nucleus on the buffer layer 42 being linked with a lateraldirection, as well as the high temperature GaN on the buffer layer bythe MOCVD method.

If the conditions of the high temperature growth and the low VI/II ratioare adopted in order to accelerate in particular the lateral growth ofthe crystalline nucleus, as shown in FIG. 6 to FIG. 9, the crystallinenucleus can be combined at an early stage, and flattening of the hightemperature grown film can be possible.

In the case of the high temperature growth using the halogenide of Zn,the reactivity of the halogenide of Zn and oxygen material is not ashigh as Zn and oxygen material. Accordingly, the high temperature growthusing the halogenide of Zn has few reactions in the vapor phase, andthere is little occurrence of particle.

This reactant lowness has had great influence also on the crystallinenucleus growth process. That is, since the crystalline nucleus does notform the film by occurring innumerably on the buffer layer 42 but thehigh temperature growth using the halogenide of Zn can grow up thesparse crystalline nucleus greatly, it can grow up effective crystallineZnO layer 46 with little grain boundaries.

(Crystallinity of ZnO Crystal)

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, a X-ray rocking curveof the (002) plane of ZnO is expressed as shown in FIG. 10. In FIG. 10,each curve expresses an example whose the crystal growth periods are 0minute, 20 minutes, 60 minutes, 180 minutes, and 360 minutes, as aparameter. The crystal growth period=0 minute is equivalent to the stateafter forming the buffer layer. In a ZnO crystal, the (002) plane is aplane vertical to a c-axis, and the X-ray rocking curve shown in FIG. 10expresses a tilt profile which is fluctuation of the c-axial directionwhich is a growth direction. It proves that the film growth thickness ofthe ZnO crystal rises, the full width at half maximum of the X-rayrocking curve of the ZnO crystal decreases, and the crystallinity of theZnO crystal having the (002) plane is improving, in accordance with thecrystal growth period passes with 20 minutes, 60 minutes, 180 minutes,and 360 minutes.

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, a X-ray rocking curveof (101) plane of ZnO is expressed as shown in FIG. 11. Each curveexpresses an example whose the crystal growth periods are 0 minute, 20minutes, 60 minutes, 180 minutes, and 360 minutes, as a parameter, sameas that of FIG. 10. The crystal growth period=0 minute is equivalent tothe state after forming the buffer layer. In the ZnO crystal, the (101)plane is a semi-polar plane, and the X-ray rocking curve shown in FIG.11 expresses a twist profile and a tilt profile which are fluctuation ofa direction vertical to the semi-polar plane. It proves that the filmgrowth thickness of the ZnO crystal rises, the full width at halfmaximum of the X-ray rocking curve of the ZnO crystal decreases, and thecrystallinity of the ZnO crystal is improving, in accordance with thepassage of time of crystal growth.

In the hetero epitaxial crystal structure formed by the hetero epitaxialgrowth method according to the first embodiment, the relation betweenthe full width at half maximum FWHM (arcsec) of the X-ray rocking curveof the ZnO crystal and the crystal growth period (min.) is expressed asshown in FIG. 12. An example of the ZnO (002) plane and an example ofthe ZnO (101) plane are shown in FIG. 12. The vertical axis of FIG. 12expresses the FWHM (Full Width at Half Maximum) (arc sec.). Here, it isequivalent to 3600 (arcsec)=1 (degree).

In the ZnO crystal by which hetero epitaxial growth is performed on thesapphire substrate, the value of full width at half maximum FWHM of therocking curve of the (101) plane and (002) plane improves in accordancewith the passage of time of crystal growth. In particular, it isdecreasing rather than 0.1 (degree)=360 (arcsec), and the crystallinityis satisfactory.

Here, in the case of this embodiment, when the crystal growth period is180 minutes, about 1.5 μm of ZnO crystal layer on the buffer layergrows, and when crystal growth period is 360 minutes, about 3 μm of ZnOcrystal layer on the buffer layer grows. Therefore, in this embodiment,as for after the crystal growth period at 180 minutes when not less thanabout 1.5 μm of ZnO crystal layer grows, the full width at half maximumFWHM becomes not more than 0.1 (degree) in the crystal (002) and (101)planes. In particular, although the (101) plane is measured as anexample of the semi-polar plane according to the first embodiment, itdoes not limit to the example. Of course about other semi-polar planes,transition of same full width at half maximum FWHM is obtained. As anexample of other semi-polar planes, there are (102) plane, (103) plane,(111) plane, (112) plane, (113) plane, (201) plane, (203) plane, (221)plane, (223) plane, etc. Moreover, in the non-polar plane (100) or (110)vertical to the (001) plane, the same full width at half maximum is alsoobtained.

(Crystal Growth Apparatus)

As shown in FIG. 13, a schematic configuration of an epitaxial crystalgrowth apparatus 20 applying to the hetero epitaxial growth methodaccording to the first embodiment includes a gaseous chlorine supplyingunit 2, a carrier gas supplying unit 3, a source zone 4, an heating unit5, a water supplying unit 6, a carrier gas supplying unit 7, a growthzone 8, an heating unit 9, and a substrate holding unit 10.

A configuration for applying to the hetero epitaxial growth method of aMgZnO semiconductor (ZnO based semiconductor) is also disposed in theepitaxial crystal growth apparatus 20 applying to the hetero epitaxialgrowth method. That is, as shown in FIG. 13, the epitaxial crystalgrowth apparatus 20 includes a gaseous chlorine supplying unit 12, acarrier gas supplying unit 13, and a source zone 14 by which group IImetallic material 25 including the single metal substance of magnesiumis held.

In the hetero epitaxial growth method of the ZnO based semiconductor(ZnO, MgZnO), the apparatus configuration for performing impurity dopingof a n type impurity or a p type impurity is also disposed on theepitaxial crystal growth apparatus 20. That is, as shown in FIG. 13, theepitaxial crystal growth apparatus 20 includes a first doping gassupplying unit 22 for supplying a n type doping gas for performingimpurity doping of the n type impurity, and a second doping gassupplying unit 24 for supplying a p type doping gas for performingimpurity doping of the p type impurity.

The source zone 4 is for holding a group II metallic material 15consisting of a zinc single metal substance. Moreover, the source zone 4is a zone for generating zinc chloride gas by reacting the gaseouschlorine, which are supplied from the gaseous chlorine supplying unit 2,and zinc. A source zone 14 is for holding magnesium of single substanceinstead of the zinc held by the source zone 4.

The growth zone 8 is a zone for growing up the ZnO semiconductor on agrowth substrate 16 held on a substrate holding unit 10 by reacting thezinc chloride gas supplied from the source zone 4 connected by asupplying pipe, and the water (vapor) supplied from the water supplyingunit 6 as an oxygen material.

In addition, each supplying pipe for connecting the source zone 4, thegrowth zone 8 and each gas supplying unit, and the growth zone 8 iscomposed by quartz glass.

The heating unit 5 is for heating the source zones 4 and 14 and thesupply route of water. The heating unit 9 is for annealing the growthzone 8. The hetero epitaxial crystal growth apparatus 20 achieves a hotwall method by the heating units 5 and 9. In addition, the heating unit5 is not limited to the illustrated configuration. For example, it iseffective also as a configuration in which a heating unit 5 a heats thesource zone 14, a heating unit 5 b anneals the source zone 4, a heatingunit 5 c heats the quartz tube of water supplying unit 6 and the carriergas supplying unit 7, and a heating unit 5 d heats the quartz tube of ntype doping gas supplying unit 22 and a p type doping gas supplying unit24, by using the heating units 5 a, 5 b, 5 c, and 5 d which are notillustrated.

The nitrogen gas supplied from the carrier gas supplying units 3, 7, and13 is for carrying the zinc chloride gas generated by the source zone 4,the water supplied from the water supplying unit 6, and the magnesiumchloride gas generated by the source zone 14 to the growth zone 8.

Next, a fabrication method of the ZnO semiconductor by the epitaxialcrystal growth apparatus 20 mentioned above will be explained.

First of all, the gaseous chlorine and the nitrogen gas are carried tothe source zone 4, respectively from the gaseous chlorine supplying unit2 and the carrier gas supplying unit 3. Then, in the source zone 4, thereaction by the following reaction formula (1) occurs by the group IImetallic material 15 consisting of the single metal substance of thezinc currently held, and the supplied gaseous chlorine, and the zincchloride gas is generated.Zn(s,l)+Cl₂(g)<=>ZnCl₂(g)  (1)In this case, as for the single metal substance of the zinc held at thesource zone 4, it is preferred that it is a single metal substance withhigh purity, for example, not less than 99.99999% of its single metalsubstance is effective. In addition, (s), (l), and (g) in the reactionformula show a solid, a fluid, and gas, respectively.

The source zone 4 becomes the structure which enlarges surface area ofthe group II metallic material 15 consisting of the zinc single metalsubstance and a suitable temperature so that most reactions in thereaction formula (1) are advanced to the right-hand side, and the flowrate of zinc chloride gas can be controlled by the amount of supply ofgaseous chlorine. As such the suitable temperature, about 300 degrees C.to about 450 degrees C. are preferable. Moreover, the temperature of thesource zone 4 is set as not more than about 500 degrees C., in order tosuppress that the zinc gas with extremely high vapor pressure also inmetal is carried to the growth zone 8. Then, the zinc chloride gasgenerated by the above-mentioned reaction formula (1) is carried to thegrowth zone 8 by the nitrogen gas supplied from the carrier gassupplying unit 3.

Moreover, the water (vapor) supplied from the water supplying unit 6 iscarried to the growth zone 8 as an oxygen material through other pathsby the nitrogen gas supplied from the carrier gas supplying unit 7.

Then, in the growth zone 8, the thin film of the ZnO semiconductor growson the growth substrate 16 with the zinc chloride gas and water whichare carried when the reaction of the following reaction formula (2)proceeds to the right-hand side.ZnCl₂(g)+H₂O(g)<=>ZnO(s)+2HCl(g)  (2)In this case, the temperature of the growth zone 8 is set as hightemperature rather than the temperature of the source zone 4 so that thezinc chloride gas may not deposit in the intermediate path to the growthzone 8. More specifically, the temperature of the growth zone 8 is setas about 500 degrees C. to about 1100 degrees C.

In the fabrication method of the MgZnO semiconductor according to theepitaxial crystal growth apparatus 20, the gaseous chlorine is carriedto the source zone 14 with carrier gas, and the magnesium chloride gasis generated in the source zone 14. Then, the MgZnO semiconductor can begrown up on the growth substrate 16 by carrying the magnesium chloridegas to the growth zone 8 by the nitrogen gas supplied from the carriergas supplying unit 13, and reacting the zinc chloride gas, the magnesiumchloride gas and the water in the growth zone 8.

In the hetero epitaxial growth method of the ZnO based semiconductor(ZnO, MgZnO), it can perform impurity doping of a desired n typeimpurity or p type impurity toward the growth layer by using the n typedoping gas supplying unit 22 and the p type doping gas supplying unit24.

As mentioned above, in the epitaxial crystal growth apparatus 20according to the first embodiment, since the single metal substance ofthe zinc with high purity instead of zinc chloride is adopted as thegroup II metallic material 15, the ZnO based semiconductor of highquality can be fabricated easily.

Moreover, it can suppress that the zinc chloride gas generated by thesource zone 4 deposits during being carried to the growth zone 8 bysetting the temperature of the source zone 4 lower than the growthtemperature of the growth zone 8.

Moreover, since not the gaseous chlorine but the hydrochloric acid gasis generated in the growth zone 8 by adopting not the single oxygensubstance but the water as an oxygen material, the reaction formula (2)can be proceeded to the right-hand side in the state where it is morestabilized thermodynamically.

For example, the single metal substance of cadmium may be adoptedinstead of the single metal substance of magnesium, as the group IImetallic material.

Moreover, oxygen gas may be adopted instead of the water as the oxygenmaterial.

Moreover, bromine gas may be adopted instead of the gaseous chlorine asthe halogen gas.

Moreover, the kind of group II metallic material may not be limited totwo kinds, and it may use more than three kinds of group II metallicmaterials.

(Formation Method for ZnO Template)

As a formation method for a ZnO template, the VPE method for merelyreacting steam of Zn with vapor will be explained hereinafter. Inaddition, in the formation method, not less than 400 degrees C. of thegrowth temperature is preferable, for example, and the more growthtemperature becomes high temperature, the more it can reflect theperformance of an underlying crystal substrate in the ZnO templatesatisfactory.

The configuration shown in FIG. 13 can be used for the crystal growthapparatus 20 also about the VPE method for merely reacting steam of Znwith the vapor, as the formation method for the ZnO template.

In the hetero epitaxial growth method according to the first embodiment,an optical microscope photograph of the ZnO buffer layer surface formedby reacting the zinc metal to the water is expressed as shown in FIG.14A, and an optical microscope photograph of the ZnO buffer layersurface formed by reacting the zinc chloride to the water is expressedas shown in FIG. 14B. In FIG. 14, the growth temperature is about 400degrees C., and the growth period is about 60 minutes. The partialpressure PH₂O of H₂O is 2.2×10⁻² atm in FIG. 14A, and the partialpressure PH₂O of H₂O is 1.32×10⁻² atm in FIG. 14B. As clearly from FIG.14B, although about 0.93 μm in thickness is obtained in the result ofthe optical microscope photograph of the ZnO buffer layer surface formedby reacting zinc chloride with water, non-homogeneous nucleation isobserved. On the other hand, although about 0.28 μm in thickness isobtained in the result of the optical microscope photograph of the ZnObuffer layer surface formed by reacting zinc metal with water, growth ofcontinuous membrane is observed.

In the hetero epitaxial growth method according to the first embodiment,the method for growing up the thin film of the ZnO semiconductor on thegrowth substrate 16 by the HVPE method after forming the ZnO bufferlayer formed by reacting zinc metal with water is expressed as follows.

(a) First of all, nitrogen gas is carried to the source zone 4 from thecarrier gas supplying unit 3, as Step 1. Then, in the source zone 4, thegroup II metallic material 15 consisting of the single metal substanceof the zinc currently held is evaporated, and is introduced in thegrowth zone 8. On the other hand, the evaporated zinc reacts with thevapor introduced in the growth zone 8 from the water supplying unit 6,and then the thin film of the ZnO semiconductor grows on the growthsubstrate 16 according to the reaction according to the followingformula (3). In this case, the crystal growth temperature of the ZnObuffer layer 42 is not less than about 400 degrees C., for example, andthe thickness is not more than about 0.3 micrometer, for example.Zn(g)+H₂O(g)<=>ZnO(s)+H₂(g)  (3)(b) Next, it anneals with the nitrogen gas in the atmosphere of thevapor H₂O(g), as Step 2. The annealing temperature is about 1000 degreesC., for example. By annealing in the manner, the crystallinity and thesurface flatness of the ZnO buffer layer 42 can be improved.(c) Next, as Step 3, the gaseous chlorine is carried to the source zone4 from the gaseous chlorine supplying unit 2, and the nitrogen gas issimultaneously carried to the source zone 4 from the carrier gassupplying unit 3. Then, in the source zone 4, the reaction according tothe reaction formula (1) occurs by the group II metallic material 15consisting of the single metal substance of the zinc currently held, andthe supplied gaseous chlorine, and the zinc chloride gas is generated.Then, the zinc chloride gas generated according to the reaction formula(1) is carried to the growth zone 8 by the nitrogen gas supplied fromthe carrier gas supplying unit 3. Moreover, the water (vapor) suppliedfrom the water supplying unit 6 is carried to the growth zone 8 as anoxygen material through other paths by the nitrogen gas supplied fromthe carrier gas supplying unit 7. Then, in the growth zone 8, the thinfilm of the ZnO semiconductor grows on the growth substrate 16 with thezinc chloride gas and the water which are carried, when the reaction ofthe reaction formula (2) proceeds to the right-hand side. Here, thecrystal growth temperature is about 1000 degrees C., for example.

In the hetero epitaxial growth method according to the first embodiment,a SEM photograph of the ZnO buffer layer surface formed by reacting zincmetal with water (Step 1) is expressed as shown, for example in FIG.15A, a SEM photograph of the ZnO buffer layer surface annealed with thenitrogen gas in the steam atmosphere (Step 2) is expressed as shown, forexample in FIG. 15 (b), and a SEM photograph on the surface of the ZnOlayer formed by reacting zinc chloride with water (Step 3) is expressedas shown, for example in FIG. 15C.

In FIG. 15A, the growth temperature is about 400 degrees C., the growthperiod is about 60 minutes, and the partial pressure P_(H2O) of H₂O isabout 2.2×10⁻² atm.

In FIG. 15B, the annealing temperature is about 1000 degrees C., theheat treating time is about 60 minutes, and the partial pressure P_(H2O)of H₂O is about 4.4×10⁻³ atm.

In FIG. 15C, the growth temperature is about 1000 degrees C., the growthperiod is about 60 minutes, the partial pressure PH₂O of H₂O is about4.4×10⁻⁴ atm, and the VI/II ratio is about 20. As shown in FIG. 15A toFIG. 15C, an aspect that the crystallinity becomes satisfactory isobserved in accordance with Steps 1-3.

In the hetero epitaxial growth method related to the first embodiment,an example of a result of a measurement of a X-ray diffractionmeasurement (2 theta-omega method) for explaining the orientation of theZnO crystal corresponding to each step of FIG. 15 is expressed as shownin FIG. 16. In FIG. 16, a vertical axis expresses XRD intensity (a.u.),and a horizontal axis expresses 2θ. It is observed that the ZnO (002)plane orients toward the a-plane sapphire substrate, and the peakintensity of ZnO (002) increases and the orientation of the crystal isimproving in accordance with shifting to Step 1 to Step 3.

In the hetero epitaxial growth method according to the first embodiment,a X-ray rocking curve (example of the ZnO (002) plane) for explainingthe crystallinity of the ZnO crystal corresponding to each step of FIG.15 is expressed as shown, for example in FIG. 17A, and a X-ray rockingcurve (example of the ZnO (101) plane) for explaining the crystallinityof the ZnO buffer layer corresponding to each step of FIG. 15 isexpressed as shown, for example in FIG. 17B.

As clearly from FIG. 17A, the value of full width at half maximum FWHM(arcsec) of the rocking curve of the (002) plane of ZnO is about 1209(arcsec) at Step 1, is about 460 (arcsec) at Step 2, and is about 327(arcsec) at Step 3. Moreover, as clearly from FIG. 17B, the value offull width at half maximum FWHM (arcsec) of the rocking curve of the(101) plane of ZnO is about 1458 (arcsec) at Step 1, is about 702(arcsec) at Step 2, and is about 493 (arcsec) at Step 3. As clear fromFIG. 17A and FIG. 17B, it proves that the twist profile and the tiltprofile of the ZnO layer oriented to (002) decrease, and thecrystallinity is improving, since the full width at half maximum of therocking curve of the (002) plane and (101) plane decreases in accordancewith shifting to Step 1 to Step 3.

(Thickness Dependency of Buffer Layer)

In the hetero epitaxial growth method according to the first embodiment,the relation between the crystal growth temperature and the growthperiod is expressed as shown in FIG. 18. The case where the growthperiod of the buffer layer is 0 (minute) is correspond to the case wherethere is no buffer layer.

(a) In FIG. 18, first of all, the a-plane sapphire substrate surface iscleaned by performing thermal cleaning (T. C.) in H₂ at high temperatureof 1000 degrees C., in the time t1 to t2.

(b) Next, the carrier gas is switched to N₂ and the buffer layer isgrown up, in the time t3 to t4, as Step 1. In order to investigate thethickness dependency of the buffer layer, the crystal growth period ofthe buffer layer is changed to 0 (minute) to 60 (minutes). The growthtemperature of the buffer layer is about 400 degrees C., the partialpressure P_(Zn) of Zn(g) is about 4×10⁻⁵ atm, and the partial pressurePH₂O of H₂O is about 4.4×10⁻³ atm.(c) Next, the annealing treatment is performed, as Step 2, in the timet5 to t6. The annealing temperature in this case is about 1000 degreesC., the annealing time is about 10 (minutes), and the partial pressurePH₂O of H₂O is about 4.4×10⁻³ atm.(d) Next, the crystal growth of the ZnO layer is performed by the HVPEmethod, as Step 3, in the time t6 to t7. The crystal growth temperaturein this case is about 1000 degrees C., the crystal growth period isabout 60 (minutes), the supplied partial pressure P_(ZnCl2) of ZnCl₂ isabout 2.2×10⁻⁵ atm, and the VI/II ratio is about 20.

A result obtained by the growing condition shown in FIG. 18 is expressedas shown in FIG. 19. FIG. 19A shows an optical microscope photograph ofthe ZnO crystal surface in the case of setting the growth period of thebuffer layer to 0 (minute), and FIG. 19B shows a bird's-eye view SEMphotograph corresponding to FIG. 19A. FIG. 19C shows an opticalmicroscope photograph of the ZnO crystal surface in the case of settingthe growth period t3 to t4 of the buffer layer to 10 (minutes), and FIG.19D shows a bird's-eye view SEM photograph corresponding to FIG. 19C.

FIG. 19E shows an optical microscope photograph of the ZnO crystalsurface in the case of setting the growth period t3 to t4 of the bufferlayer to 30 (minutes), and FIG. 19F shows a bird's-eye view SEMphotograph corresponding to FIG. 19E.

FIG. 19G shows an optical microscope photograph of the ZnO crystalsurface in the case of setting the growth period t3 to t4 of the bufferlayer to 60 (minutes), and FIG. 19H shows a bird's-eye view SEMphotograph corresponding to FIG. 19G.

As a result of investigating the state of the crystal surface of the ZnOlayer which grows by the HVPE method on the buffer layer by changing thethick film (growth period) of the buffer layer formed by low-temperaturegrowth of about 400 degrees C., as clearly from FIG. 19A to FIG. 19H,the state of the crystal surface of the ZnO layer is as satisfactory asthe film thickness of the buffer layer being thick (growth period beinglong). It is considered because the two-dimensional growth of the ZnOlayer by the HVPE method is as accelerated as the film thickness of thebuffer layer being thick (growth period being long).

A SEM photograph of the crystal surface of the ZnO layer in the case ofsetting the growth period of the buffer layer on the a-plane sapphiresubstrate to 20 minutes and setting the growth period of the ZnO layerto 20 minutes is expressed as shown in FIG. 20A, and an expansionbird's-eye view SEM photograph corresponding to FIG. 20A is expressed asshown in FIG. 20B. Moreover, a SEM photograph of the crystal surface ofthe ZnO layer in the case of setting the growth period of the bufferlayer on the a-plane sapphire substrate to 60 minutes and setting thegrowth period of the ZnO layer to 20 minutes is expressed as shown inFIG. 20C, and an expansion bird's-eye view SEM photograph correspondingto FIG. 20C is expressed as shown in FIG. 20D.

As clearly from FIG. 20A to FIG. 20D, a growth mode, by which ahexagon-shaped growth nucleus is generated on the crystal surface of theZnO layer formed on the buffer layer formed at low temperature, thegrowth nucleus of the shape of the hexagon combines with each other, andflattening is performed, is achieved. Moreover, the lateral growth ofthe ZnO layer formed by the HVPE method on the buffer layer is asaccelerated as the thickness of the buffer layer formed at lowtemperature being thick.

(X-ray Diffraction Measurement (2 Theta-Omega Method))

In the hetero epitaxial growth method according to the first embodiment,a result of a measurement of X-ray diffraction measurement (2theta-omega method) of the crystal of the ZnO layer after performing thecrystal growth of the ZnO layer by the HVPE method for 60 minutes afterformation of the buffer layer on the a-plane sapphire substrate for 10minutes is expressed as shown, for example in FIG. 21. Similarly, aX-ray diffraction result of a measurement after performing the crystalgrowth of the ZnO layer by the HVPE method for 60 minutes after theformation of the buffer layer for 30 minutes is expressed as shown, forexample in FIG. 22. Furthermore, a X-ray diffraction result of ameasurement after performing the crystal growth of the ZnO layer by theHVPE method for 60 minutes after the formation of the buffer layer for60 minutes is expressed as shown, for example in FIG. 23. The result ofFIG. 21 corresponds to FIG. 19C and FIG. 19D, the result of FIG. 22corresponds to FIG. 19E and FIG. 19F, and the result of FIG. 23corresponds to FIG. 19G and FIG. 19H.

As shown in FIG. 21, when the thickness of the buffer layer formed bylow-temperature growth is thin, the orientation of the ZnO layer formedby the HVPE method on the buffer layer is not match, and ZnO (101) planeexcept a desired ZnO (002) plane are detected. On the other hand, asshown in FIG. 22 and FIG. 23, when the thickness of the buffer layerformed by low-temperature growth becomes thick, the orientation of theZnO layer formed by the HVPE method on the buffer layer becomessatisfactory, and the desired ZnO (002) plane is detected.

(X-ray Rocking Curve)

In the hetero epitaxial growth method according to the first embodiment,examples of the X-ray rocking curve after performing the crystal growthof the ZnO layer (ZnO (002) plane) by the HVPE method on the bufferlayer for 60 minutes after performing the formation of the buffer layeron the a-plane sapphire substrate for 30 minutes and 60 minutes,respectively, are expressed as shown in FIG. 24A in the (002) plane andexpressed as shown in FIG. 24B in the (101) plane.

As clearly from FIG. 24A, the values of full width at half maximum FWHM(arcsec) of the rocking curve of the (002) plane of ZnO are 396 (arcsec)and 327 (arcsec), respectively, when the buffer layer is grown for 30minutes and 60 minutes. Similarly, as clearly from FIG. 24B, the valuesof full width at half maximum FWHM (arcsec) of the rocking curve of the(101) plane of ZnO are 504 (arcsec) and 493 (arcsec), respectively, whenthe buffer layer is grown for 30 minutes and 60 minutes.

The full width at half maximum FWHM (arcsec) of the XRD rocking curve ofthe ZnO layer which became a film is as satisfactory as not more than500 (arcsec) by accelerating two dimensional crystal growth as shown inFIG. 24A and FIG. 24B, and the reduction of the value of the full widthat half maximum of the rocking curve can be expected with the furtherincrease in the film growth thickness as well as FIG. 12.

(X-Ray Phi Scan)

In the hetero epitaxial growth method according to the first embodiment,a X-ray phi scan of the c-axis oriented ZnO layer formed on the a-planesapphire substrate 40 is performed by setting incident X ray and a X-raydetector, and rotating centering on a c-axial direction, so that the ZnO(101) plane may be detected, as shown in FIG. 25. A result of ameasurement of the X-ray phi scan of a ZnO layer 46 by the existence ornonexistence of a buffer layer 42 formed on an a-plane sapphiresubstrate 40 is expressed as shown in FIG. 26. When there is no bufferlayer 42 (WITHOUT LT-Buffer), as the dashed line A in FIG. 26 other thana great peak symmetrical with 6 times shows, the crystalline nucleus inthe case of rotating 30 degrees is observed, and the crystallinity ofthe direction of twist which is the direction of Φ is disordered. On theother hand, when there is the buffer layer 42 (WITH LT-Buffer), thehexagonal 6 times symmetry of ZnO layer 46 is observed exactly.Moreover, when there is the buffer layer 42, the crystalline nucleus inthe case of rotating 30 degrees is not observed, but it is effective inmatching crystallinity also in the direction of twist which is thedirection of Φ.

(r-Plane Sapphire Substrate)

In the hetero epitaxial growth method according to the first embodiment,a schematic cross-section structure of ZnO layer 47 performed thecrystal growth, without forming the buffer layer, on the r-planesapphire substrate 41 is expressed as shown in FIG. 27A. Moreover, asurface bird's-eye view SEM photograph after performing the crystalgrowth of ZnO layer 47 by the HVPE method for 60 minutes, withoutforming the buffer layer, on the r-plane sapphire substrate 41 isexpressed as shown in FIG. 27B. As shown in FIG. 27A and FIG. 27B, asfor the shape of ZnO layer 47 performed the crystal growth, withoutforming a buffer layer, on the r-plane sapphire substrate 41, thehexagonal pillar crystal is observed.

On the other hand, in the hetero epitaxial growth method according tothe first embodiment, a schematic cross-section structure of ZnO layer47 performed the crystal growth, after forming the buffer layer 43, onthe r-plane sapphire substrate 41 is expressed as shown in FIG. 28A.Moreover, a surface bird's-eye view SEM photograph after performing thecrystal growth of ZnO layer 47 by the HVPE method for 60 minutes afterforming the buffer layer 43 on the r-plane sapphire substrate 41 isexpressed as shown in FIG. 28B. An effect is found in planarizingformation of ZnO layer 47 by the HVPE method deposited on the bufferlayer by the ZnO buffer layer 43 not only on the a-plane sapphiresubstrate 40, but also on the r-plane sapphire substrate 41.

In the hetero epitaxial growth method according to the first embodiment,a surface bird's-eye view SEM photograph performed the crystal growth ofthe buffer layer 43 at 400 degrees C. for 60 minutes on the r-planesapphire substrate 41 is expressed as shown in FIG. 29A, and a surfacebird's-eye view SEM photograph of the ZnO layer further performed thecrystal growth at 1000 degrees C. for 60 minutes is expressed as shownin FIG. 29B. As clearly from FIG. 29, an effect is found in planarizingformation of ZnO layer 47 by the HVPE method deposited on the bufferlayer by the ZnO buffer layer 43 also on the r-plane sapphire substrate41.

A result of a measurement of X-ray diffraction measurement (2theta-omega method) of the ZnO layer formed in FIG. 29 is expressed asshown, for example in FIG. 30. Since the peak of ZnO (110) is observedas shown in FIG. 30, the ZnO layer 47 which performs a-axis oriented isformed on the r-plane sapphire substrate 41.

(Semiconductor Device)

As shown in FIG. 31, an example of schematic cross-section structure ofa semiconductor device having the hetero epitaxial crystal structureformed by the hetero epitaxial growth method according to the firstembodiment includes: a heterogeneous substrate 40; a buffer layer 42disposed on the heterogeneous substrate 40; a n type ZnO basedsemiconductor layer 52 disposed on the buffer layer 42 andimpurity-doped with the n type impurity; a ZnO based semiconductoractive layer 54 disposed on the n type ZnO based semiconductor layer 52;and a p type ZnO based semiconductor layer 56 disposed on the ZnO basedsemiconductor active layer 54 and impurity-doped with the p typeimpurity.

Each of the n type ZnO based semiconductor layer 52, the ZnO basedsemiconductor active layer 54, and the p type ZnO based semiconductorlayer 56 are formed by the above-mentioned hetero epitaxial crystalgrowth apparatus 20, and the hetero epitaxial growth method used for theabove-mentioned halogenation group II metal and oxygen material.

A p side electrode 60 is disposed on the p type ZnO based semiconductorlayer 56, and a n side electrode 70 is disposed on the surface of the ntype ZnO based semiconductor layer 52 exposed by mesa etching. As amaterial of the p side electrode 60, the layered structure of a Ni layerand an Au layer is adoptable, for example. Moreover, as a material ofthe n side electrode 70, the layered structure of a Ti layer and an Aulayer is adoptable, for example.

Moreover, in the example shown in FIG. 31, in order that the extractionefficiency of the light from the ZnO based semiconductor active layer 54is raised, as for the back side 40 b of the heterogeneous substrate 40,uneven random shape, such as a surface roughening process, is formed.

In addition, as the n type impurity supplied with the n type doping gas,any one of B, Ga, Al, In or Tl is applicable, for example.

Moreover, as the p type impurity supplied with the p type doping gas,any one of N, P, As, Sb, Bi, Li, or Cu is applicable, for example.

The ZnO based semiconductor active layer 54 includes a MQW(Multi-Quantum Well) structure where a barrier layer composed ofMg_(x)Zn_(1-x)O (where 0<x<1) and a well layer composed of ZnO aredeposited, for example.

Alternatively, the ZnO based semiconductor active layer 54 may include aMQW structure where a well layer composed of Cd_(y)Zn_(1-y)O (where0<y<1) and a barrier layer composed of ZnO is deposited.

The number of pairs of the quantum well is decided from the transitdistance of electrons and holes. That is, it determines by the number ofpairs of MQW corresponding to the predetermined thickness of the ZnObased semiconductor active layer 54 from which the recombinationradiation efficiency of electrons and holes becomes the mostsatisfactory.

Here, since the bandgap energy of MgO is 7.8 eV in contrast to thebandgap energy of ZnO is 3.37 eV, a MQW structure by which a barrierlayer composed of Mg_(x)Zn_(1-x)O, and a well layer composed of ZnO aredeposited can be formed, by adjusting the composition ratio x ofMg_(x)Zn_(1-x)O.

On the other hand, since the bandgap energy of CdO is 0.8 eV in contrastto the bandgap energy of ZnO is 3.37 eV, a MQW structure by which a welllayer composed of Cd_(y)Zn_(1-y)O, and a barrier layer composed of ZnOare deposited can also be formed, by adjusting the composition ratio yof Cd_(y)Zn_(1-y)O.

The heterogeneous substrate 40 is composed of a sapphire substrate, forexample. Alternatively, a silicon substrate, a SiC substrate, a GaAssubstrate, a GaP substrate, a GaN based substrate, or a ScAlMgO₄substrate may be applied.

As the buffer layer 42, an orienting film of an oxide or an orientingfilm of nitride is applicable. As the orienting film of the oxide, ZnOor MgO can be used, for example.

As the orienting film of the nitride, AlN or GaN can be used, forexample.

In addition, the light-emitting (hν) from the above-mentionedsemiconductor device can be extracted from the direction of a topsurface and a back side, as shown in FIG. 31. As a final devicestructure, it is effective also as a configuration by which thelight-emitting (hν) is mainly extracted from the back side 40 b of theheterogeneous substrate 40, for example by making it as a flip-chipmounting structure.

According to the hetero epitaxial growth method according to the presentinvention, the ZnO based semiconductor crystal can be formed on theheterogeneous substrate, such as a sapphire substrate, at thetemperature higher than 800 degrees C.

According to the present invention, the hetero epitaxial crystalstructure and the semiconductor device, which are formed by using theabove-mentioned hetero epitaxial growth method, can be provided.

According to the hetero epitaxial crystal growth apparatus according tothe present invention, the ZnO based semiconductor crystal can be grownup on the heterogeneous substrate, such as a sapphire substrate, at thetemperature higher than 800 degrees C.

Second Embodiment Homo Epitaxial Growth Method

A homo epitaxial growth method of a homo epitaxial crystal structureaccording to a second embodiment of the present invention will beexplained with reference to FIG. 32.

A schematic cross-section structure showing a process for preparing aZnO substrate 40 is expressed as shown in FIG. 32A. A schematiccross-section structure showing a formation process of a ZnO basedsemiconductor layer 46 by halogenide vapor phase epitaxy is expressed asshown in FIG. 32B.

(a) First of all, as shown in FIG. 32A, +c-plane (001) substrate isprepared as the ZnO substrate 40, for example. A crystal plane isprovided with the plane where the c-axis fine-inclined about 0.5 degreein the m-axial direction <1-10>, for example.

(b) Next, as shown in FIG. 32B, the ZnO substrate 40 is heated to thehigh temperature of not less than about 1000 degrees C., and the ZnObased semiconductor layer 46 is formed on the ZnO substrate 40 by theHVPE method. More specifically, the reactant gas mixing zinc containinggas and oxygen containing gas is introduced on the ZnO substrate 40, andthe crystal growth of the ZnO based semiconductor layer 46 is performedon the ZnO substrate 40. For example, the halogenide vapor phase epitaxyusing zinc chloride (ZnCl₂) and water (H₂O) is applied.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set to about not more than 1×10⁻⁴ atm, for example, the VI/IIratio which is a supply ratio of oxygen which is group VI element and Znwhich is group II element is set to not more than about 100, forexample, the crystal growth temperature T_(g) is set to about 1000degrees C., for example, and the crystal growth period is set to about 1to 6 hours, for example. Here, in the case of the partial pressurePZnCl₂ of ZnCl₂=1×10⁻⁴ atm, it will be set to the partial pressureP_(H2O) of H₂O=10⁻² atm if the supply ratio VI/II=100.

As a value of the VI/II ratio which is the supply ratio of oxygen whichis group VI element and Zn which is group II element, it is preferablethat it is more than 1 and not more than about 100, for example.

In the homo epitaxial growth method according to the second embodiment,the temperature T_(g) of the crystal growth is enforcing the hightemperature growth method higher than 1000 degrees C. If the meltingpoint of 1975 degrees C. of the ZnO crystal is taken into consideration,about ⅓ to ½ of the melting point is needed, and more the temperatureT_(g) of crystal growth is high, the more the satisfactory crystal ofquality can be obtained. Therefore, in the above-mentioned example, thesame growth of degree 1000 degrees C. as GaN is performed.

In the process of crystal growth of the ZnO based semiconductor layer46, the reactant gas may further include magnesium containing gas.Moreover, the partial pressure of magnesium containing gas is not morethan about 1×10⁻⁴ atm, for example.

In the process of the crystal growth of the ZnO based semiconductorlayer 46, it is preferable that the crystal growth temperature is notless than 1000 degrees C.

In the process of the crystal growth of the ZnO based semiconductorlayer 46, it may further include a process of supplying the impurity gascomposed of a halogenide of gallium or aluminum.

In this case, the n type semiconductor layer in which the ZnO basedsemiconductor layer 46 has the carrier concentration not less than about1×10¹⁶ cm⁻¹, for example may be formed.

(Temperature Dependence of Surface Morphology of ZnO Based SemiconductorLayer)

In a homo epitaxial crystal structure (crystal growth temperatureT_(g)=700 degrees C.) formed by the above-mentioned homo epitaxialgrowth method, an AFM photograph of the surface of the ZnO basedsemiconductor layer of a part of 2 μm×2 μm is expressed as shown in FIG.33A, and an AFM photograph of the ZnO based semiconductor layer of awide range part of 20 μm×20 μm is expressed as shown in FIG. 33B.Similarly, an example of the crystal growth temperature T_(g)=800degrees C. is expressed as shown in FIG. 34A and FIG. 34B, an example ofthe crystal growth temperature T_(g)=900 degrees C. is expressed asshown in FIG. 35A and FIG. 35B, and an example of the crystal growthtemperature T_(g)=1000 degrees C. is expressed as shown in FIG. 36A andFIG. 36B.

As compared with the organic metal material used for the conventionalMOCVD method, the halogenide of zinc and/or magnesium is not decomposedinto zinc and/or magnesium and gaseous chlorine at degree 1000 degreesC. high temperature, but the halogenide of zinc and/or magnesium andoxygen material are directly reacted on the ZnO substrate.

As for the halogenide of zinc and/or magnesium and oxygen material,since a premature reaction does not occur like main organic metals andoxygen material, the raw material efficiency of the high temperatureregion is also higher than the MOCVD method, without particle occurringon the ZnO substrate.

(VI/II Ratio Dependence of Surface Morphology of ZnO Based SemiconductorLayer)

In a homo epitaxial crystal structure (example of crystal growthtemperature T_(g)=1000 degrees C. and VI/II ratio=20) formed by the homoepitaxial growth method according to the second embodiment, an AFMphotograph of the surface of the ZnO based semiconductor layer of a partof 2 μm×2 μm is expressed as shown in FIG. 37A, and an AFM photograph ofthe ZnO based semiconductor layer of a wide range part of 20 μm×20 μm isexpressed as shown in FIG. 37B. Similarly, an example of the crystalgrowth temperature T_(g)=1000 degrees C. and the VI/II ratio=50 isexpressed as shown in FIG. 38A and FIG. 38B, and an example of thecrystal growth temperature T_(g)=1000 degrees C. and the VI/IIratio=1000 is expressed as shown in FIG. 39A and FIG. 39B.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set to 2.2×10⁻⁵ atm, the crystal growth temperature T_(g) is1000 degrees C., and the VI/II ratio is changed to 20 to 1000.

The mol supply ratio (VI/II ratio) of the oxygen materials and thehalogenide of zinc and/or magnesium is also an important growthparameter. By setting the VI/II ratio to not more than 100, themigration on the substrate surface of halide materials is accelerated,and it is effective in reducing the occurrence of the abnormality part(a pit or a projection) of crystal growth, etc. Moreover, since thespeed of crystal growth becomes slow when the VI/II ratio is small, theVI/II ratio is effective to apply more than 1.

(Growth Period Dependence of Surface Morphology of ZnO BasedSemiconductor Layer)

In a homo epitaxial crystal structure (example of crystal growthtemperature T_(g)=1000 degrees C., VI/II ratio=20, crystal growthperiod=1 hour) formed by the homo epitaxial growth method according tothe second embodiment, an AFM photograph of the surface of the ZnO basedsemiconductor layer of a part of 2 μm×2 μm is expressed as shown in FIG.40A, and an AFM photograph of the ZnO based semiconductor layer of awide range part of 20 μm×20 μm is expressed as shown in FIG. 40B.Similarly, an example of the crystal growth temperature T_(g)=1000degrees C., the VI/II ratio=20 and the crystal growth period=2 hours isexpressed as shown in FIG. 41A and FIG. 41B, and an example of thecrystal growth temperature T_(g)=1000 degrees C., the VI/II ratio=20 andthe crystal growth period=6 hours is expressed as shown in FIG. 42A andFIG. 42B.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set to 2.2×10⁻⁵ atm, the VI/II ratio is set to 20, the crystalgrowth temperature T_(g) is 1000 degrees C., and the crystal growthperiod is changed to 1 hour to 6 hours.

The film growth thickness of the ZnO based semiconductor layer 46 isabout 0.42 μm, for example in 1 hour of crystal growth periods, is about0.73 μm, for example in 2 hours of crystal growth periods, and is about2.27 μm, for example in 6 hours of crystal growth periods.

Although the vapor phase epitaxial crystal growth using the halogenideis known as a method in which the high-speed growth which is severaltens to several hundred μm per hour is possible, the growth becomespossible with the growth rate of not more than about 1 μm per hour byreducing the material partial pressure to the usual MOCVD method level,and it prevents forming excessive nucleus formation and excessivepolycrystalline film by reducing the material partial pressure.

The value of the not more than about 1 μm per hour is a growth ratepermissible as a thin-film formation rate of compound semiconductor, andis fully usable for a method as the formation method of the ZnO basedsemiconductor layer.

(1 ML Step)

In the homo epitaxial crystal structure formed by the homo epitaxialgrowth method according to the second embodiment, an AFM photographshowing the surface morphology having a step of 1 ML (Mono Layer) heightof the surface of the ZnO based semiconductor layer 46 having +c-planein alignment with a m-axial direction <1-10> and an a-axial direction<110> is expressed as shown in FIG. 43.

Furthermore, in the homo epitaxial crystal structure formed by the homoepitaxial growth method according to the second embodiment, an enlargeddrawing of an AFM photograph showing the surface morphology having astep of 1 ML height of the surface of the ZnO based semiconductor layer46 having +c-plane in alignment with the m-axial direction <1-10> andthe a-axial direction <110> is expressed as shown in FIG. 44A. Moreover,a schematic planar structure of the step shape of 1 ML stepcorresponding to FIG. 44A is expressed as shown in FIG. 44B. Moreover, aschematic cross-section structure of the step shape between point A andpoint B corresponding to FIG. 44B can be schematically expressed asshown in FIG. 44C. The surface of the ZnO based semiconductor layerinclines about 0.5 degree to the m-axial direction <1-10>. Furthermore,a drawing showing the m-axial direction <1-10> and a-axial direction<110> which assumed the hexagonal shape on the ZnO based semiconductorlayer surface corresponding to FIG. 44A is expressed as shown in FIG.44D.

As a result of the surface observation of FIG. 44A, the stripes in linewith the <110> directions are observed, and about 13 pieces exist on thescreen of FIG. 44A. Furthermore, a wavelike curve is observed betweensuch the stripes. Such the shape expresses a result in which every 1 MLof steps have appeared as well as the (2×1) structure observed in thefine inclined plane (direction of incline is [110], angle of gradient is0.5) in the (001) plane of silicon. That is, as shown in FIG. 44B, sevenstripes exist between the point A and the point B, and the heightdifference of each stripe is equivalent to 2 ML. The interval of thestripe and the wavelike curve is equivalent to 1 ML.

The terrace width w in the case of 2 ML step is expressed as thefollowing formula:w=h/tan θ  (4)where h is the lattice constant of the c-axial direction of the ZnOcrystal, and θ expresses the angle to which the c-axis of the substrateinclined from the normal line of the substrate to the m-axial direction.In an example of FIG. 44, θ=0.5 degree. As a result of FIG. 44, itproves that the terrace width w is 0.0597 μm and h=0.5207 (nm) isobtained.

The surface performing a step flow in a wide area is obtained by settingthe growth temperature to not less than 1000 degrees C., setting thesupplied partial pressure of the zinc halogenide to not more than 1×10⁻⁴atm and setting the VI/II ratio to not more than 100, and the surface of1 ML step (half of the lattice constant of the c-axis of ZnO) isobtained when further grown up at 1000 degrees C.

It is first time to have observed 1 ML step surface by the vapor phaseepitaxy film of ZnO. The pit is not observed by the AFM observation ofwide range of 20 μm×20 μm, and the pit density is not more than 1×10cm⁻² same as the dislocation density of the substrate also as a resultof the microscopic observation. It proves that the pit-free ZnO basedsemiconductor layer is formed covering a wide area compared with otherprocesses such as the MOCVD method.

(Crystallinity of ZnO Crystal)

—X-Ray Rocking Curve—

In the homo epitaxial crystal structure formed by the homo epitaxialgrowth method according to the second embodiment, a X-ray rocking curveof the ZnO crystal having the (002) plane is expressed as shown in FIG.45. In FIG. 45, each curve expresses the ZnO substrate 40 and the ZnObased semiconductor layer 46 as a parameter. In the ZnO crystal, the(002) plane is a plane perpendicular to the c-axis, and the X-rayrocking curve shown in FIG. 45 expresses the tilt profile which isfluctuation of the c-axial direction. In the homo epitaxial crystalstructure formed by the homo epitaxial growth method according to thesecond embodiment, it proves that a result with the full width at halfmaximum FWHM (arcsec) of the X-ray rocking curve of the ZnO crystalhaving the (002) plane substantially equivalent to the full width athalf maximum FWHM of the ZnO substrate is obtained. That is, it provesthat the full width at half maximum FWHM of the ZnO based semiconductorlayer 46 having the (002) plane is 18.7 (arcsec) in contrast to the fullwidth at half maximum FWHM=17.3 (arcsec) of the ZnO substrate having the(002) plane, and the crystallinity of the ZnO crystal having the (002)plane is satisfactory.

In the homo epitaxial crystal structure formed by the homo epitaxialgrowth method according to the second embodiment, the X-ray rockingcurve of the ZnO crystal having the (101) plane is expressed as shown inFIG. 46. Each curve expresses the ZnO substrate 40 and the ZnO basedsemiconductor layer 46 as a parameter as well as FIG. 45. In the ZnOcrystal, the (101) plane is a semi-polar plane and the X-ray rockingcurve shown in FIG. 46 expresses the twist profile and tilt profilewhich is fluctuation of a direction vertical to the semi-polar plane. Inthe homo epitaxial crystal structure formed by the homo epitaxial growthmethod according to the second embodiment, it proves that a result withthe full width at half maximum FWHM of the X-ray rocking curve of theZnO crystal having the (101) plane substantially equivalent to the fullwidth at half maximum FWHM of the ZnO substrate is obtained. That is, itproves that the full width at half maximum FWHM of the ZnO basedsemiconductor layer 46 having the (101) plane is 13.0 (arcsec) incontrast to the full width at half maximum FWHM=12.2 (arcsec) of the ZnOsubstrate 40 having the (101) plane, and the crystallinity of the ZnOcrystal having the (101) plane is satisfactory.

—SIMS Analysis Result—

A SIMS (Secondary Ion-microprobe Mass Spectrometry) result (polarity ofdetected ion: −) of the ZnO crystal of the homo epitaxial crystalstructure formed by the homo epitaxial growth method according to thesecond embodiment is expressed as shown in FIG. 47. Similarly, a SIMSanalysis result (polarity of detected ion: +) of the ZnO crystal isexpressed as shown in FIG. 48. In FIG. 47 and FIG. 48, a vertical axisshows secondary ion strength (count), and a horizontal axis shows mass(AMU). The detected ion of FIG. 47 is an example for irradiating O₂ ⁻ion as irradiated ion, and the detected ion of FIG. 48 is an example fordetecting Cs⁺ ion as irradiated ion.

As clearly from the result of FIG. 47 and FIG. 48, it proves thatchlorine of the halogen based, etc. is not detected and chlorine doesnot contain in the ZnO crystal of the homo epitaxial crystal structureformed by the homo epitaxial growth method according to the secondembodiment.

A SIMS analysis result showing the depth direction dependence of thechlorine concentration of the ZnO crystal of the homo epitaxial crystalstructure formed by the homo epitaxial growth method according to thesecond embodiment is expressed as shown in FIG. 49. In FIG. 49, avertical axis expresses the chlorine concentration (cm⁻³) and ahorizontal axis expresses the depth (μm). Near the interface of the ZnOsubstrate 40 and the ZnO based semiconductor layer 46, the chlorideconcentration is on the detection limit level of chlorine substantially.

As clearly from the result of FIG. 49 showing the depth directiondependence, it proves substantially that the chlorine does not containat the homo epitaxial crystal structure formed by the homo epitaxialgrowth method according to the second embodiment.

Every element except the component element of the ZnO basedsemiconductor layer 46/ZnO substrate 40 from the impurity measurement bythe SIMS analysis result is a detection limit, and therefore it provesthat the homo epitaxial growth method according to the second embodimentis suitable for fabrication of the ZnO semiconductor film enough by nothaving detected the halogenide in particular.

—Electrical Characteristics—

In a result of a measurement of electrical characteristics of the ZnOfilm of the homo epitaxial crystal structure formed by the homoepitaxial growth method according to the second embodiment, an exampleof C-V characteristics of the MOS structure is expressed as shown inFIG. 50. Moreover, as shown in FIG. 51, a schematic cross-sectionstructure of a sample applying to C-V measurement of FIG. 50 includes: aZnO substrate 40; a ZnO based semiconductor layer 46 disposed on the ZnOsubstrate 40; an insulating film 48 disposed on the ZnO basedsemiconductor layer 46; a MOS electrode 50 disposed on the insulatingfilm 48; and a substrate electrode 38 electrically connected with theZnO substrate 40. The thickness of the ZnO based semiconductor layer 46is 2.3 μm, and the thickness of the insulating film 48 is 0.2 μm. Thematerial of the MOS electrode 50 is composed of layered structure ofTi/Au, and the diameter is 100 μm.

In FIG. 50, the vertical axis expresses the capacitance value of MOScapacitors (F/cm²), and the horizontal axis expresses the voltage (V)for applying to a forward direction and an opposite direction. The smallsignal frequency at the time of the measurement is 100 kHz. As clearlyfrom FIG. 50, as for the CV curve, since the depletion region and theinversion region have appeared in the minus voltage application sidebordering on 0 V neighborhoods, and the accumulation region has appearedin the plus voltage application side, it proves that the grown-up ZnOfilm is a n-type semiconductor.

As shown in FIG. 50, since 2.38×10⁻⁸ (F/cm²) is obtained as for themaximum capacitance C_(max), 4.4×10⁻⁹ (F/cm²) is obtained as for theminimum capacitance C_(min), and the residual carrier concentration isabout 1×10¹⁵ cm⁻³, it proves that the ZnO based semiconductor layer 46of high resistivity is formed.

Here, the residual carrier concentration can be obtained as follows.

As shown in FIG. 51, in the MOS capacitance structure composed of MOSelectrode 50/insulating film 48/ZnO based semiconductor layer 46/ZnOsubstrate 40/substrate electrode 38, the maximum depletion layer widthW_(max) (μm) is expressed such as the following formula as aone-dimensional model.W _(max)={4∈_(s) kTln(N _(D) /n _(i))/(q ² N _(D))}^(1/2)  (5)

The minimum capacitance C_(min) (F/cm²) is expressed such as thefollowing formula:

$\begin{matrix}\begin{matrix}{C_{\min} = {ɛ_{i}/\left\lbrack {d + {\left( {ɛ_{i}/ɛ_{s}} \right)W_{\max}}} \right\rbrack}} \\{= {ɛ_{i}/\left\lbrack {d + \left\{ {4ɛ_{i}^{2}k\; T\;{{\ln\left( {N_{D}/n_{i}} \right)}/\left( {ɛ_{s}q^{2}N_{D}} \right)}} \right\}^{1/2}} \right\rbrack}}\end{matrix} & (6)\end{matrix}$where, k expresses the Boltzmann's constant, and T expresses theabsolute temperature. As the relative dielectric constant of ZnO∈_(s)=8.5, the relative dielectric constant of the insulating film 48∈_(i)=5.0, the film thickness of the insulating film 48 d=0.2 (μm), andthe intrinsic carrier concentration of ZnO n_(i)=1×10⁻⁸, the relationbetween the minimum capacitance C_(min) (F/cm²) and the maximumdepletion layer width W_(max) (μm) for the carrier concentration N_(D)(cm⁻³) calculated from the formula (5) and the formula (6) is expressedas shown in FIG. 52.

As clearly from FIG. 50, it proves that 4.4×10⁻⁹ (F/cm²) is obtained asfor the value of the minimum capacitance C_(min), the carrierconcentration N_(D) is about 1×10¹⁵ cm⁻³ corresponding to the minimumcapacitance C_(min)=4.4×10⁻⁹ (F/cm²), and the maximum depletion layerwidth W_(max) is about 2 μm.

According to the electrical property evaluation, it proves that theresidual donor concentration is about 1×10¹⁵ cm⁻³, is low enough as theresidual donor concentration of the compound semiconductor, and thecarrier concentration not less than 1×10¹⁵ cm⁻³ can be controlled bydoping the impurity doping gas. Since the halogenide is used for mainmaterials, it is possible to control the doping quantity stable when thehalogenide, in particular halogenide of gallium or halogenide ofaluminum when doping the n type impurity, is supplied also to the dopinggas.

(Crystal Growth Apparatus)

A schematic configuration of an epitaxial crystal growth apparatus 20applying to the homo epitaxial growth method according to the secondembodiment is similarly expressed as FIG. 13, and includes a gaseouschlorine supplying unit 2, a carrier gas supplying unit 3, a source zone4, an heating unit 5, a water supplying unit 6, a carrier gas supplyingunit 7, a growth zone 8, an heating unit 9, and a substrate holding unit10.

A configuration for applying to the homo epitaxial growth method of theMgZnO semiconductor (ZnO based semiconductor) is also disposed at theepitaxial crystal growth apparatus 20 applying to the homo epitaxialgrowth method. That is, as shown in FIG. 13, the epitaxial crystalgrowth apparatus 20 includes a gaseous chlorine supplying unit 12, acarrier gas supplying unit 13, and a source zone 14 by which the groupII metallic material 25 including the single metal substance ofmagnesium is held.

In the homo epitaxial growth method of the ZnO based semiconductor (ZnO,MgZnO), an equipment configuration for impurity-doping the n typeimpurity or the p type impurity is also disposed at the epitaxialcrystal growth apparatus 20. That is, as shown in FIG. 13, the epitaxialcrystal growth apparatus 20 includes a first doping gas supplying unit22 for supplying the n type doping gas for impurity-doping the n typeimpurity, and a second doping gas supplying unit 24 for supplying the ptype doping gas for impurity-doping the p type impurity.

The source zone 4 is for holding a group II metallic material 15consisting of a zinc metal single substance. Moreover, the source zone 4is a zone for generating zinc chloride gas by reacting the gaseouschlorine which are supplied from the gaseous chlorine supplying unit 2and zinc.

The growth zone 8 is a zone for growing up the ZnO semiconductor on agrowth substrate 16 held on a substrate holding unit 10 by reacting thezinc chloride gas supplied from the source zone 4 connected by thesupplying pipe, and the water (vapor) supplied from the water supplyingunit 6 as an oxygen material. In addition, hydrogen for adjusting thedriving force of crystal growth is supplied to the growth zone 8 from ahydrogen supplying unit (not shown). Since the hydrogen supplied fromthe hydrogen supplying unit reacts to gaseous chlorine easily, thehydrogen is supplied to the growth zone 8 in a supplying path differentfrom the supplying path of gaseous chlorine.

In addition, each supplying pipe for connecting the source zone 4, thegrowth zone 8, and each gas supplying unit and the growth zone 8 iscomposed by quartz glass.

The heating unit 5 is for heating the source zones 4 and 14 and thesupplying path of water. The heating unit 9 is for heating the growthzone 8. The homo epitaxial crystal growth apparatus 20 achieves a hotwall method by the heating units 5 and 9.

In addition, the heating units 5 may be individually provided to thesource zones 4 and 14. Moreover, the heating units 5 may be individuallydisposed by heater wires etc. also to each gas supplying path.

The nitrogen gas supplied from the carrier gas supplying units 3, 7, and13 is for carrying the zinc chloride gas generated by the source zone 4,the water supplied from the water supplying unit 6, and the magnesiumchloride gas generated by the source zone 14 to the growth zone 8.

The growth zone 8 may be formed with a hot wall type reactor.

The second group II metallic material includes magnesium, for example.

As the n type doping gas, it is effective to supply the halogenide ofgallium or aluminum.

Next, a fabrication method of the ZnO semiconductor by the epitaxialcrystal growth apparatus 20 mentioned above will be explained.

First of all, the gaseous chlorine and the nitrogen gas are carried tothe source zone 4, respectively from the gaseous chlorine supplying unit2 and the carrier gas supplying unit 3. Then, in the source zone 4, thereaction by the following reaction formula (7) occurs by the group IImetallic material 15 consisting of the single metal substance of thezinc currently held, and the supplied gaseous chlorine, and the zincchloride gas is generated.Zn(s,l)+Cl₂(g)<=>ZnCl₂(g)  (7)In this case, as for the single metal substance of the zinc held at thesource zone 4, it is preferred that it is a single metal substance withhigh purity, for example, not less than 99.99999% of its single metalsubstance is effective. In addition, (s), (l), and (g) in the reactionformula show a solid, a liquid, and gas, respectively.

The source zone 4 becomes the structure enlarging the surface area ofthe group II metallic material 15 composed of zinc single metalsubstance, and a suitable temperature, so that most reactions in thereaction formula (7) may be proceeded to the right-hand side and theflow rate of zinc chloride gas can be controlled by the amount of supplyof gaseous chlorine. In addition, as such the suitable temperature,about 300 degrees C. to about 450 degrees C. are preferable. Moreover,the temperature of the source zone 4 is set as not more than about 500degrees C., in order to suppress that the zinc gas with extremely highvapor pressure also in metal is carried to the growth zone 8. Then, thezinc chloride gas generated by the above-mentioned reaction formula (7)is carried to the growth zone 8 by the nitrogen gas supplied from thecarrier gas supplying unit 3.

Moreover, the water (vapor) supplied from the water supplying unit 6 iscarried to the growth zone 8 as an oxygen material through other pathsby the nitrogen gas supplied from the carrier gas supplying unit 7.

Then, in the growth zone 8, the thin film of the ZnO semiconductor growson the growth substrate 16 with the zinc chloride gas and water whichare carried when the reaction of the following reaction formula (8)proceeds to the right-hand side.ZnCl₂(g)+H₂O(g)<=>ZnO(s)+2HCl(g)  (8)

Here, the temperature of the growth zone 8 is set as high temperaturerather than the temperature of the source zone 4, so that the zincchloride gas may not deposit in a halfway of the path to the growth zone8. More specifically, the temperature of the growth zone 8 is set asabout 500 degrees C. to about 1100 degrees C.

In the fabrication method of the MgZnO semiconductor according to thehomo epitaxial crystal growth apparatus 20, the gaseous chlorine iscarried to the source zone 14 with carrier gas, and the magnesiumchloride gas is generated in the source zone 14. Then, the MgZnOsemiconductor can be grown up on the growth substrate 16 by the nitrogengas supplied from the carrier gas supplying unit 13 carrying magnesiumchloride gas to the growth zone 8, and reacting the zinc chloride gas,the magnesium chloride gas and the water in the growth zone 8.

In the homo epitaxial growth method of the ZnO based semiconductor (ZnO,MgZnO), it can perform impurity doping of a desired n type impurity or ptype impurity toward the growth layer by using the n type doping gassupplying unit 22 and the p type doping gas supplying unit 24.

In addition, the present inventor has verified in thermodynamicanalyzing that it is possible to proceed the reaction of the reactionformula (8) to the right-hand side also at high temperature (forexample, not less than about 1000 degrees C.).

As mentioned above, in the homo epitaxial crystal growth apparatus 20according to the second embodiment, since the single metal substance ofthe zinc with high purity instead of zinc chloride is adopted as thegroup II metallic material 15, the ZnO based semiconductor of highquality can be fabricated easily.

Moreover, it can suppress depositing by the time the zinc chloride gasgenerated in the source zone 4 is carried to the growth zone 8 bysetting the temperature of the source zone 4 lower than the growthtemperature of the growth zone 8.

Moreover, since not the gaseous chlorine but the hydrochloric acid gasis generated in the growth zone 8 by adopting not the single oxygensubstance but the water as an oxygen material, the reaction formula (8)can be proceeded to the right-hand side in the state where it is morestabilized thermodynamically.

For example, the single metal substance of cadmium may be adoptedinstead of the single metal substance of magnesium, as the group IImetallic material.

Moreover, oxygen gas may be adopted instead of the water as the oxygenmaterial.

Moreover, bromine gas may be adopted instead of the gaseous chlorine asthe halogen gas.

Moreover, the kind of group II metallic material may not be limited totwo kinds, and it may use more than three kinds of group II metallicmaterials.

(Semiconductor Device)

As shown in FIG. 53, an example of a schematic cross-section structureof a semiconductor device having the homo epitaxial crystal structureformed by the homo epitaxial growth method according to the secondembodiment includes: a ZnO substrate 40; a n type ZnO basedsemiconductor layer 52 disposed on the ZnO substrate 40 andimpurity-doped with the n type impurity; a ZnO based semiconductoractive layer 54 disposed on the n type ZnO based semiconductor layer 52;and a p type ZnO based semiconductor layer 56 disposed on the ZnO basedsemiconductor active layer 54 and impurity-doped with the p typeimpurity.

Each of the n type ZnO based semiconductor layer 52, the ZnO basedsemiconductor active layer 54, and the p type ZnO based semiconductorlayer 56 are formed by the above-mentioned homo epitaxial crystal growthapparatus 20, and the homo epitaxial growth method used for theabove-mentioned halogenation group II metal and oxygen material.

A p side electrode 60 is disposed on the p type ZnO based semiconductorlayer 56, and a n side electrode 70 is disposed at the back side of theconductive ZnO substrate 40. As a material of the p side electrode 60,the layered structure of a Ni layer and a Au layer is adoptable, forexample. Moreover, as a material of the n side electrode 70, the layeredstructure of a Ti layer and a Au layer is adoptable, for example.

As a n type impurity, any one of B, Ga, Al, In, or T¹ is applicable, forexample.

Moreover, as a p type impurity, any one of N, P, As, Sb, Bi, Li, or Cuis applicable, for example.

The ZnO based semiconductor active layer 54 includes a MQW structure bywhich a barrier layer composed of Mg_(x)Zn_(1-x)O (where 0<x<1), and awell layer composed of ZnO are deposited, for example.

Alternatively, the ZnO based semiconductor active layer 54 may include aMQW structure by which a well layer composed of Cd_(y)Zn_(1-y)O (where0<y<1), and a barrier layer composed of ZnO are deposited.

The number of pairs of the quantum well is decided from the transitdistance of electrons and holes. That is, it determines by the number ofpairs of MQW corresponding to the predetermined thickness of the ZnObased semiconductor active layer 54 from which the recombinationradiation efficiency of electrons and holes becomes the mostsatisfactory.

Here, since the bandgap energy of MgO is 7.8 eV in contrast to thebandgap energy of ZnO is 3.37 eV, the MQW structure by which the barrierlayer composed of Mg_(x)Zn_(1-x)O, and the well layer composed of ZnOare deposited can be formed, by adjusting the composition ratio x ofMg_(x)Zn_(1-x)O.

On the other hand, since the bandgap energy of CdO is 0.8 eV in contrastto the bandgap energy of ZnO is 3.37 eV, the MQW structure by which thewell layer composed of Cd_(y)Zn_(1-y)O, and the barrier layer composedof ZnO are deposited can also be formed, by adjusting the compositionratio y of Cd_(y)Zn_(1-y)O.

In addition, the light-emitting (hν) from the above-mentionedsemiconductor device can be extracted from the direction of a topsurface, as shown in FIG. 53. As a final device structure, it iseffective also as a configuration by which the light-emitting (hν) ismainly extracted from the back side of the ZnO substrate 40, for exampleby making it as a flip-chip mounting structure.

According to the homo epitaxial growth method according to the presentinvention, the ZnO based semiconductor crystal can be formed on the ZnOsubstrate at the temperature higher than 1000 degrees C.

According to the present invention, the homo epitaxial crystal structureand the semiconductor device which are formed by using theabove-mentioned homo epitaxial growth method can be provided.

According to the homo epitaxial crystal growth apparatus according tothe present invention, the ZnO based semiconductor crystal can be grownup on the ZnO substrate at the temperature higher than 1000 degrees C.

Third Embodiment Fabrication Method of ZnO Based Semiconductor

A fabrication method of a ZnO based semiconductor according to a thirdembodiment of the present invention will explained with reference toFIG. 32 since it is expressed same as FIG. 32.

First of all, as shown in FIG. 32A, a ZnO substrate in which the surfacehaving +c-plane (001) is prepared as a substrate 40, for example. Acrystal plane is provided with the plane where the c-axis fine-inclinedabout 0.5 degree in the m-axial direction <1-10>, for example.

Next, as shown in FIG. 32B, the ZnO substrate 40 is heated to hightemperature at about 700 degrees C. to about 1000 degrees C., and a ZnObased semiconductor layer 46 by which the p type impurity (acceptorelement) is doped is formed on the ZnO substrate 40 by the HVPE method.

More specifically, on the ZnO substrate 40, the reactant gas mixing thehalide gas which zinc contained, and the oxygen containing gas isintroduced, and the hydride gas of the group V (nitrogen group) actingas p type impurity material gas is also introduced. Thus, the crystalgrowth of the p type ZnO based semiconductor layer 46 is performed onthe ZnO substrate 40.

For example, ammonia (NH₃) is used for the p type impurity material gaswhen applying the p type impurity as nitrogen. Moreover, halogenidevapor phase epitaxy is performed by using zinc chloride (ZnCl₂) andwater (H₂O) for the reactant gas.

In addition, AsH₃ or PH₃ etc. acting as the hydride gas, such as As, P,etc. which are group V elements can be used for group V hydride gas.

As the crystal growth conditions, the partial pressure P_(ZnCl2) ofZnCl₂ is set to not more than about 1×10⁻⁴ atm, for example, the VI/IIratio which is a supply ratio (partial pressure ratio) between the gascontaining oxygen which is group VI element, and the halide gascontaining Zn which is group II element is set to not more than about100, for example, the crystal growth temperature T_(g) is set to about1000 degrees C., for example, and the crystal growth period is set toabout 1 to 6 hours, for example. Here, in the case of the partialpressure P_(ZnCl2) of ZnCl₂=1×10⁻⁴ atm, for example, it is set to thepartial pressure P_(H2O) of H₂O=10⁻² atm if the supply ratio VI/II isset to 100.

As the value of the VI/II ratio which is the supply ratio between oxygenwhich is group VI element and Zn which is group II element, it ispreferable that it is more than 1 and not more than about 100, forexample.

On the other hand, when NH₃ is used for the group V hydride gas actingas p type impurity material gas, it is known that the NH₃ gas becomes amore effective nitrogen source even if the NH₃ gas is not plasmaized asused for the nitride growth with the MBE method or the CVD.

The present inventors found out that enough nitrogen could be doped bymaking the p type impurity material gas into a hydride gas of group Vmaterial and reacting to halide gas including Zn, even if the growthtemperature is set to high. Furthermore, it has verified that thecrystallinity improved when the ZnO of the p type impurity doping isgrown up by the above-mentioned HVPE method, even if it is a growthtemperature region to which the crystallinity reduces by undoped ZnOetc. conventionally.

An AFM image of the surface in the case of performing the crystal growthof the undoped ZnO layer by the HVPE method on the ZnO substrate in therange whose growth temperature is 800 degrees C. to 900 degrees C. isshown in FIG. 54. As shown in FIG. 54, hexagonal pyramid-shaped pitsoccur.

On the other hand, a typical surface AFM image of a nitrogen doped ZnOlayer sample is shown in FIG. 55 as the ZnO based semiconductor formedby using the fabrication method of the ZnO based semiconductor accordingto the third embodiment. More specifically, on the ZnO substrate 40,ZnCl₂ is introduced as halide gas, H₂O is introduced as oxygencontaining gas, NH₃ is introduced as hydride gas of the group V actingas the p type impurity material gas, and then the nitrogen doped ZnObased semiconductor is formed. That is, the crystal growth of the p typeZnO based semiconductor layer 46 is performed on the ZnO substrate 40.The growth temperature of halogenide vapor phase epitaxy is set to 800degrees C., the partial pressure P_(ZnCl2) of ZnCl₂ is set to 2.2×10⁻⁵atm, the partial pressure P_(H2O) of H₂O is set to 4.4×10⁻⁴ atm, and thepartial pressure P_(NH3) of NH₃ is set to 4.4×10⁻⁴ atm.

As proved in FIG. 55, step bunching occurred by introducing NH₃ and thenthe pits have disappeared. That is, even if it is the growth temperaturewhich the pit occurs on the undoped ZnO layer grown up on the ZnOsubstrate, it proves that the crystallinity improves if the nitrogendoped ZnO layer is applied, and it becomes usable.

Next, the NH₃ gas is effective as the nitrogen source of the plasma lessin the vapor phase epitaxy, and in order to increase the nitrogen dopedquantity, it is considered that what is necessary is just to usuallyincrease the flow rate of NH₃ gas, i.e., to increase the partialpressure PNH₃ of NH₃. However, the following inconvenience occurs inother side. Since NH₃ gas includes hydrogen, the hydrogen occurred bydecomposition of a part of NH₃ such as the following reaction formula(9) reduces the growth driving force of ZnO. This is because thehydrogen reduces ZnO.NH₃→(N₂/2)+(3H₂/2)  (9)

FIG. 56A shows the relation between the crystal growth driving force andthe growth temperature in the case of changing the VI/II ratio which isa partial pressure ratio (supply ratio) between the H₂O containingoxygen of the group VI, and the halide gas ZnCl₂ of Zn of group II basedon a thermodynamic analysis result. A vertical axis shows the crystalgrowth driving force (atm), and a horizontal axis shows the growthtemperature (degrees C.).

The partial pressure P_(ZnCl2) of ZnCl₂ is set to 2.2×10⁻⁵ atm.Moreover, the V/II ratio which is the partial pressure ratio between theNH₃ which is hydride gas of group V and the halide gas ZnCl₂ of Zn ofgroup II is set to 20. The decomposition rate of NH₃ on the substrate isset to 3%. As proved in FIG. 56A, the more the value of the VI/II ratiobecomes small (i.e., the more the amount of supply of H₂O decreases),the more growth driving force is reduced. Moreover, it proves that themore the growth temperature becomes high, the more the growth drivingforce decreases.

On the other hand, FIG. 56B is a drawing for extracting two greatlydifferent curves whose VI/II ratios are 1000 and 10 among FIG. 56A, andis enlarged and displayed with the dashed lines. Although each the V/IIratio of two above-mentioned curves extracted from FIG. 56A are 20, fulllines shows the curve as the curve A and the curve B in the case ofsetting the flow rate of NH₃ to 0 and setting the V/II ratio to 0. Asproved in the curve A and the curve B, when there is no NH₃ flow, thecrystal growth driving force is not reduced even if raising the growthtemperature. However, when there is NH₃ flow, the reduction in drivingforce is immediately seen for the side where the VI/II ratio is lowereven if the growth temperature is remarkably low. As proved in FIG. 56B,the region where driving force is reduced appears with any values in theVI/II ratio if the growth temperature rises by introducing NH₃.

As mentioned above, if the decomposition rate of NH₃ is assumed, it ispossible to determine the possibility of the growth in a certain growthtemperature and material partial pressure based on the thermodynamicanalysis, and it is possible to predict beforehand the conditions intowhich the nitrogen doped ZnO layer 46 is grown up according to FIG. 56Aand FIG. 56B.

On the other hand, as shown hereinafter, it proved that the growthdriving force is reduced and the etching occurs conversely if the growthtemperature is set to high under the fixed partial pressure of NH₃ alsofrom a result of the experiment.

FIG. 57 shows how the crystal growth driving force actually changesaccording to the partial pressure P_(NH3) of NH₃ gas which is hydridegas of group V. The partial pressure P_(ZnCl2) of halide gas ZnCl₂ of Znof group II is set to 2.2×10⁻⁵ atm, and the partial pressure P_(H2O) ofH₂O containing oxygen of group VI is set to 4.4×10⁻⁴ atm, in thehalogenide vapor phase epitaxy. Moreover, Partial pressure P_(NH3) ofNH₃ is changed to 4.4×10⁻³ atm, 4.4×10⁻⁴ atm, and 4.4×10⁻⁵ atm. Thegrowth temperature is changed to 700 degrees C., 800 degrees C., 900degrees C., and 1000 degrees C. In FIG. 57, the vertical axis shows thegrowth rate of the ZnO crystal (μm/h), and the horizontal axis shows thegrowth temperature (degrees C.).

The range in which growth rate is larger than 0 (positive range) showsplus growth, and the range whose growth rate is smaller than 0 (negativerange) shows minus growth, i.e., it is etched. It proves that the morepartial pressure P_(NH3) of NH₃ becomes large, the more the etching hasstarted immediately from a part with low growth temperature.

A drawing in which the data of FIG. 57 is summarized intelligibly isFIG. 58. FIG. 58 is a drawing showing whether the plus growth isperformed or it is etched (minus growth), about each point ofmeasurement of the growth temperature is 700 degrees C., 800 degrees C.,900 degrees C., and 1000 degrees C., regarding to the curve of threekinds of partial pressure of NH3 of FIG. 57. It is made intelligible bydescribing “∘” to the plus growth and describing “x” to the minus growth(etching).

As shown in FIG. 57 and FIG. 58, the growth temperature by which theetching is started is reduced in accordance that the partial pressure ofNH3 becomes large.

FIG. 59 shows the analysis result of the depth direction of the nitrogen(N) concentration by the SIMS measurement of the nitrogen doped ZnOlayer 46 which sets the partial pressure P_(ZnCl2) of ZnCl₂ to 2.2×10⁻⁵atm, sets the partial pressure P_(H2O) of H₂O to 4.4×10⁻⁴ atm, and setsthe partial pressure P_(NH3) of NH₃ gas to 4.4×10⁻⁵ atm, changes thegrowth temperature in the range of 700 degrees C. to 900 degrees C., andperforms the crystal growth on the ZnO substrate 40, by the HVPE method.The horizontal axis shows the depth (μm) from the surface of thenitrogen doped ZnO layer 46, and the vertical axis shows N (nitrogen)concentration (cm⁻³).

As proved in FIG. 59, when the partial pressure P_(NH3) of NH₃ is set to4.4×10⁻⁵ atm, the plus growth is performed in the growth temperature of700 degrees C. to 900 degrees C. In the state where the plus growth isperformed, the N concentration becomes the highest in the nitrogen dopedZnO layer 46 at highest 900 degrees C. of the growth temperature, andthe N concentration also decreases sequentially in accordance that thegrowth temperature decrease to 800 degrees C. and to 700 degrees C.

Thus, when the nitrogen doped ZnO based semiconductor is fabricated bythe fabrication method of the ZnO based semiconductor according to thethird embodiment, it proves that there is positive temperaturecorrelation that the nitrogen concentration increases in accordance withthe growth temperature rise.

In the fabrication method of the ZnO based semiconductor according tothe third embodiment, since the nitrogen doping concentration becomeshigh and the activation of nitrogen becomes easy to be performed in thecase of high temperature growth in the condition that the partialpressure P_(NH3) of NH₃ is fixed, it is extremely preferable forapplying the p type.

Moreover, as seen in FIG. 55, although there is an effect of thecrystalline improvement by the nitrogen dope, the crystallineimprovement is further expectable by performing the high temperaturegrowth.

Moreover, since the reduction effect for the ZnO in the side where thepartial pressure PNH₃ of NH₃ is higher becomes strong, the reductioneffect for the ZnO appears in the more low temperature side if thepartial pressure P_(NH3) of NH₃ becomes high. However, since it can growup by the more high temperature side and the reduction effect for theZnO can be reduced in addition to improvement in the nitrogen dopingconcentration, crystalline improvement, improvement in the nitrogenactivation, etc. if the partial pressure P_(NH3) of NH₃ is decreased, itis extremely preferable.

Next, FIG. 60 shows the concentration of the depth direction of N(nitrogen) and H (hydrogen) by the SIMS measurement of the nitrogendoped ZnO layer 46 grown up on the ZnO substrate 40. The nitrogen dopedZnO layer 46 was formed by setting the growth temperature to 800 degreesC., setting the partial pressure P_(ZnCl2) of ZnCl₂ to 2.2×10⁻⁵ atm,setting the partial pressure P_(H2O) of H₂O to 4.4×10⁻⁴ atm, and settingthe partial pressure P_(NH3) of NH₃ gas to 4.4×10⁻⁴ atm, by the HVPEmethod. The horizontal axis shows the depth (μm) from the surface of thenitrogen doped ZnO layer 46, and the vertical axis shows theconcentration (cm⁻³) of N (nitrogen) or H (hydrogen). Although N of thebackground level is contained in the ZnO substrate 40, it proves that Nof the concentration exceeding 1×10²⁰ cm⁻³ is contained in the nitrogendoped ZnO layer 46 grown up under the partial pressure P_(NH3) of NH₃gas.

Referring to FIG. 58, the limiting temperature at the side of the hightemperature of plus growth of partial pressure P_(NH3) of NH₃ gas of4.4×10⁻⁴ atm is 800 degrees C. Thus, high nitrogen concentration can beobtained in the nitrogen doped ZnO based semiconductor by applying thelimiting temperature at the side of the high temperature of the plusgrowth under the predetermined partial pressure P_(NH3) of NH₃ gas.

Consideration of FIG. 57 to FIG. 60 will draw the following tendencies.The nitrogen doped ZnO based semiconductor formed by the fabricationmethod of the ZnO based semiconductor according to the third embodimentis fabricated in the temperature range TW where the plus crystal growthis performed.

There is a relation with reverse between the V/II ratio and the limitingtemperature Th at the side of the high temperature of the temperaturerange TW where the plus crystal growth is performed. That is, thelimiting temperature Th becomes lower in accordance that the V/II ratiobecomes larger.

On the other hand, the limiting temperature Th becomes high inaccordance that the V/II ratio becomes small. Moreover, the nitrogendoping concentration becomes larger in accordance that the growthtemperature becomes high in the temperature span TW where the pluscrystal growth is performed, under the fixed V/II ratio.

Moreover, what is necessary is just to perform as follows to control todiffer the nitrogen concentration in each film as layered structurecomposed of multilayer films in the ZnO based semiconductor with whichthe p type impurity is doped. In the condition that the V/II ratio isfixed, the growth temperature of a first nitrogen doped ZnO based thinfilm in the temperature range TW of plus growth is set to T1.

Next, the growth temperature of a second nitrogen doped ZnO filmfabricated in the temperature range TW of plus growth is set to T2(T1≠T2).

Thus, if the growth temperature is changed one after another, themultilayer film in which the nitrogen concentration differs can beformed.

The multilayer film in which the p type impurity concentration(nitrogen) has an inclination can be composed by whether each growthtemperature from the growth temperature T1 of the first nitrogen dopedZnO based thin film to the growth temperature TN of the N^(th) nitrogendoped ZnO based thin film is raised sequentially, or each growthtemperature is reduced sequentially.

(Fabricating Apparatus)

FIG. 61 shows a schematic configuration of a fabricating apparatus usedfor the fabrication method of the ZnO based semiconductor according tothe third embodiment of the present invention. Although theconfiguration of FIG. 61 is substantially equivalent as the epitaxialcrystal growth apparatus according to the first embodiment of thepresent invention, detailed structure is disclosed in FIG. 61. As shownin FIG. 61, in order to form the ZnO semiconductor of the p typeimpurity doping, the fabricating apparatus includes a gaseous chlorinesupplying unit 17, a steam gas supplying unit 11, an ammonia gassupplying unit 23, a source zone 4, gas supply lines 32, 34, 36, 18 a,and 18 b, an heating unit 5, a growth zone 8, an heating unit 9, and asubstrate holding unit 10, etc. N₂ (nitrogen gas) as carrier gas ismixed in the steam gas supplying unit 11, the gaseous chlorine supplyingunit 17, and the ammonia gas supplying unit 23, as shown in FIG. 61. Agroup II metallic material 15 composed of single metal substance of thezinc of group II is held in the source zone 4.

It is also possible to add other source zones to the configuration, tothe fabricating apparatus of the ZnO based semiconductor. For example,when performing the crystal growth of MgZnO semiconductor, as shown inFIG. 61, it adds the configuration of a gaseous chlorine supplying unit21, a source zone 14, and gas supply lines 22 a and 22 b, etc. A groupII metallic material 25 including the single metal substance ofmagnesium is held in the source zone 14.

Moreover, as shown in FIG. 61, it is also possible to add a n typeimpurity material gas supply unit 13 in order to fabricate the n typeZnO based semiconductor layer. The halogenide of Ga or Al can be usedfor the n type impurity material gas supply unit 13, for example, asshown in FIG. 61, it can be applied to GaCl₃. The N₂ gas is mixed ascarrier gas also in the n type impurity material gas supply unit 13.

The source zone 4 is a zone for generating zinc chloride gas by reactingthe zinc disposed inside and the gaseous chlorine supplied through thegas supply line 18 a from the gaseous chlorine supplying unit 17, andfor supplying the zinc chloride gas to the growth zone 8 through the gassupply line 18 b.

The growth zone 8 is a zone for growing up the ZnO based semiconductoron a growth substrate 16 held on the substrate holding unit 10 byreacting the zinc chloride gas supplied from the source zone 4 connectedto the gas supply line 18 b and the water (vapor) supplied through thegas supply line 32 from the steam gas supplying unit 11 as an oxygenmaterial. In addition, being held on the substrate holding unit 10 alsohas the case of a semiconductor instead of the growth substrate. Forexample, the layered structure by which the n type ZnO basedsemiconductor layer is formed on the substrate may be held.

In addition, all the gas supply lines 32, 34, 36, 18 a, 18 b, 22 a, and22 b connecting each the gas supplying unit, the growth zone 8, and thesource zones 4 and 14 mutually are composed by quartz glass. Thus, sincethe nonmetallic quartz glass is used for the gas supply lines and thehalogenide including Zn in the metallic material of the ZnO basedsemiconductor is used, the metal for functioning as a catalyst of theNH₃ decomposition which is a hydride of group V does not exist aroundthe growth substrate 16 or the semiconductor on the substrate holdingunit 10.

The heating unit 5 is for heating the source zones 4 and 14, the gassupply lines 32, 34, 36, 18 a, 18 b, 22 a, and 22 b, etc. The heatingunit 9 is for heating the growth zone 8. The fabricating apparatus ofthe ZnO based semiconductor achieves the hot wall method by the heatingunits 5 and 9. In addition, the heating unit 5 may be individuallydisposed toward the source zones 4 and 14 and each gas supply path.

Next, the fabrication method of the ZnO based semiconductor by thefabricating apparatus of the ZnO based semiconductor mentioned abovewill be explained in detail.

First of all, gaseous chlorine and nitrogen gas are supplied to thesource zone 4 through the gas supply line 18 a from the gaseous chlorinesupplying unit 17. Then, in the source zone 4, the reaction by thefollowing reaction formulas occurs by the group II metallic material 15composed of single metal substance of the zinc currently held, and thesupplied gaseous chlorine, and zinc chloride gas is generated.Zn(s,l)+Cl₂(g)<=>ZnCl₂(g)  (10)

In this case, as for the single metal substance of the zinc held at thesource zone 15, it is preferred that it is a single metal substance withhigh purity, for example, not less than 99.99999% of its single metalsubstance is effective. In addition, (s), (l), and (g) in the reactionformula show a solid, a liquid, and gas, respectively.

The source zone 4 becomes the structure enlarging the surface area ofthe group II metallic material 15 composed of zinc single metalsubstance, and a suitable temperature, so that most reactions in thereaction formula (10) may be proceeded to the right-hand side and theflow rate of zinc chloride gas can be controlled by the amount of supplyof gaseous chlorine.

In addition, as such the suitable temperature, about 300 degrees C. toabout 450 degrees C. are preferable. Moreover, the temperature of thesource zone 4 is set as not more than about 500 degrees C., in order tosuppress that the zinc gas with extremely high vapor pressure also inmetal is carried to the growth zone 8. Then, the zinc chloride gasgenerated by the above-mentioned reaction formula (10) is carried to thegrowth zone 8 through the supplying pipe 18 b.

On the other hand, the water (vapor) supplied from the steam gassupplying unit 11 is carried to the growth zone 8 as the oxygen materialthrough the gas supply line 32. Moreover, the ammonia gas suppliedthrough the gas supply line 36 from the ammonia gas supplying unit 23 iscarried to the growth zone 8 as the nitrogen material acting as a p typeimpurity.

Then, in the growth zone 8, when the reaction by the following reactionformula proceeds to the right-hand side with the carried zinc chloridegas and water, the thin film of the ZnO semiconductor grows on thesubstrate 40.ZnCl₂(g)+H₂O(g)<=>ZnO(s)+2HCl(g)  (11)

At this time, when it reacts to NH₃ simultaneously carried on the growthsubstrate 16 or the semiconductor currently held on the substrateholding unit 10, and a part of oxygen atoms of ZnO replace nitrogen, thenitrogen doped ZnO semiconductor layer is formed.

In this case, as shown in FIG. 61, the gas supply lines 32, 34, and 36are formed to the inner side of the growth zone 8, and each gassupplying outlet port 32 a, 34 a, and 36 a is composed as disposed justabove the growth substrate 16 or the semiconductor.

By the way, as for the ammonia gas which is a kind of the hydride gas ofgroup V, although decomposition to nitrogen and hydrogen may occur fromthermodynamic analysis at least about 400 degrees C., if thedecomposition occurs and then the ammonia gas is became to N₂ and H₂, itwill not function as a nitrogen source in the case of the p typeimpurity doping.

However, even if it is not plasmaized as the NH₃ gas is used for thenitride growth by the MBE method, becoming a more effective source of Nis known. This is because almost all gas can supply also at the hightemperature of 1000 degrees C. in the environment where a catalyst suchas metal does not exist, without resolving to the substrate face. Inaddition, the H₂ gas generated from the NH₃ gas degrades the ZnO by thefollowing formula.ZnO(s)+H₂(g)<=>Zn(g)+H₂O(g)  (12)

If the H₂ partial pressure of the growth zone becomes high or the growthtemperature becomes high, the reaction by the formula (12) proceeds tothe right-hand side, and the growth of ZnO is prevented.

In the fabricating apparatus shown in FIG. 61, the nonmetallic quartzglass is used for the material of the gas supply line 36 for carryingthe ammonia gas at least, the gas supply line 36 crosses the inside ofthe growth zone 8, and the gas supplying outlet port 36 a is providedjust above the growth substrate 16. Accordingly, since the ammonia gasdoes not degrade because of being not contacted to the metal until theammonia gas reaches on the growth substrate 16, and the ammonia gas canbe contributed to the reaction just above the growth substrate 16, thesufficient nitrogen doping concentration can be obtained.

On the other hand, since the gas supplying outlet port 32 a of the gassupply line 32 for carrying the oxygen containing gas is provided justabove the growth substrate 16, and the halogenide including Zn in themetallic material for crystal growth is used and the halogenide andoxygen containing gas react just above the growth substrate 16.Accordingly, the Zn material can be supplied on the substrate veryefficiently.

In this case, the temperature of the growth zone 8 is set as hightemperature rather than the temperature of the source zones 4 and 14, sothat the zinc chloride gas may not deposit in a halfway of the path tothe growth zone 8. More specifically, the temperature of the growth zone8 is set as about 700 degrees C. to about 1000 degrees C.

Moreover, it can suppress depositing of the zinc chloride gas andmagnesium chloride gas generated in the source zones 4 and 14 is carriedto the growth zone 8 by setting the temperature of the source zones 4and 14 lower than the growth temperature of the growth zone 8.

Moreover, since not gaseous chlorine but hydrochloric acid gas isgenerated on the growth substrate 16 of the growth zone 8 by adoptingnot the oxygen simple substance but the water as the oxygen material,the ZnO thin film can be formed in the state where it is stabilized morethermodynamically.

As the hydride gas of group V, either of AsH₃ or PH₃ can be used insteadof NH₃, for example. Moreover, bromine gas may be adopted instead of thegaseous chlorine as the halogen gas. Moreover, the kind of group IImetallic material may not be limited to two kinds, and it may use morethan three kinds of group II metallic materials.

In the process of crystal growth of the ZnO based semiconductor layer,the reactant gas may further include magnesium containing gas. Moreover,the partial pressure of magnesium containing gas is not more than about1×10⁻⁴ atm, for example.

The halogenide of zinc and/or magnesium is not resolved into the zincand/or magnesium and the gaseous chlorine at the temperature of about1000 degrees C., but the direct reaction occurs with the halogenide ofzinc and/or magnesium and the oxygen material on the ZnO substrate.

The raw material efficiency of the high temperature region of thehalogenide of zinc and/or magnesium and the oxygen material is alsohigher than the MOCVD method, without particle occurring on the ZnOsubstrate, since the premature reaction does not occur like the mainorganic metals and oxygen material.

The mol supply ratio (VI/II ratio) of the oxygen materials and thehalogenide of zinc and/or magnesium is an important growth parameter.

As it explained in the second embodiment, by setting the VI/II ratio tonot more than 100, the migration on the substrate surface of halidematerials is accelerated, and it is effective in reducing the occurrenceof the abnormality part (a pit and a projection) of crystal growth, etc.Moreover, since the speed of crystal growth becomes slow when the VI/IIratio is small, the VI/II ratio is effective to apply more than 1, forexample.

For example, in the case where the partial pressure P_(ZnCl2) of ZnCl₂is 2.2×10⁻⁵ atm, and VI/II is 20, the growth rate of about 0.5 μm/h isobtained. The value of the about 0.5 μm/h is a growth rate permissibleas the thin-film formation speed of the compound semiconductor, and canfully be used as the formation method of the ZnO based semiconductorlayer.

Next, an example of schematic cross-section structure of thesemiconductor element formed by using the fabrication method and thefabricating apparatus of the above-mentioned ZnO based semiconductor issimilarly expressed as FIG. 53, and includes: a ZnO substrate 40; a ntype ZnO based semiconductor layer 52 disposed on the ZnO substrate 40and doped with the n type impurity; a ZnO based semiconductor activelayer 54 disposed on the n type ZnO based semiconductor layer 52; and ap type ZnO based semiconductor layer 56 disposed on the ZnO basedsemiconductor active layer 54, and doped with the p type impurity.

Each of the n type ZnO based semiconductor layer 52, the ZnO basedsemiconductor active layer 54, and the p type ZnO based semiconductorlayer 56 are formed by the fabrication method using the above-mentionedfabricating apparatus, and using the above-mentioned halogenation groupII metal and oxygen materials of the ZnO based semiconductor.

A p side electrode 60 is disposed on the p type ZnO based semiconductorlayer 56, and a n side electrode 70 is disposed at the back side of theconductive ZnO substrate 40. As a material of the p side electrode 60,the layered structure of a Ni layer and a Au layer is adoptable, forexample. Moreover, as a material of the n side electrode 70, the layeredstructure of a Ti layer and a Au layer is adoptable, for example.

As a n type impurity, any one of B, Ga, Al, In or Tl is applicable, forexample.

Moreover, as a p type impurity, the group V element can be used, forexample, N, P, or As etc. can be applied.

The ZnO based semiconductor active layer 54 includes a MQW structure bywhich the barrier layer composed of Mg_(x)Zn_(1-x)O (where 0<x<1), andthe well layer composed of ZnO(s) are deposited, for example.

Moreover, the ZnO based semiconductor active layer 54 may include a MQWstructure by which a well layer composed of Cd_(y)Zn_(1-y)O (where0<y<1), and a barrier layer composed of ZnO are deposited.

The number of pairs of the quantum well is decided from the transitdistance of electrons and holes. That is, it determines by the number ofpairs of MQW corresponding to the predetermined thickness of the ZnObased semiconductor active layer 54 from which the recombinationradiation efficiency of electrons and holes becomes the mostsatisfactory.

Here, since the bandgap energy of MgO is 7.8 eV in contrast to thebandgap energy of ZnO is 3.37 eV, the MQW structure by which the barrierlayer composed of Mg_(x)Zn_(1-x)O, and the well layer composed of ZnOare deposited can be formed, by adjusting the composition ratio x ofMg_(x)Zn_(1-x)O.

On the other hand, since the bandgap energy of CdO is 0.8 eV in contrastto the bandgap energy of ZnO is 3.37 eV, the MQW structure by which thewell layer composed of Cd_(y)Zn_(1-y)O, and the barrier layer composedof ZnO are deposited can also be formed, by adjusting the compositionratio y of Cd_(y)Zn_(1-y)O.

In addition, the light-emitting (hν) from the above-mentionedsemiconductor device can be extracted from the direction of a topsurface, as shown in FIG. 53.

The p type ZnO based semiconductor layer 56 of the ZnO basedsemiconductor element of FIG. 53 can also be applied as a multilayerfilm, as shown in FIG. 62, without applying a single layer. In FIG. 62,the p type ZnO based semiconductor layer 56 is composed of a p type ZnObased semiconductor layer 561, a p type ZnO based semiconductor layer562 . . . , a p type ZnO based semiconductor layer 56(n−1), and a p typeZnO based semiconductor layer 56 n. Where, n is 1 or more integers.Although the group V element which is the p type impurity may be dopedby fixed concentration, the p type ZnO based semiconductor layer 561 tothe p type ZnO based semiconductor layer 56 n can also be doped bydifferent concentration.

As described in explanation of FIG. 58 to FIG. 61, the multilayer filmin which the concentration of the group V element which is the p typeimpurity differs can be formed by fixing the V/II ratio which is a ratiobetween the partial pressure of the hydride gas of group V and thepartial pressure of the halogenation group II metallic gas includingzinc, and changing the growth temperature one after another for everysemiconductor layer in the temperature span of the crystal growth ofplus of the p type ZnO based semiconductor layer. Moreover, if thegrowth temperature is raised or reduced sequentially from the p type ZnObased semiconductor layer 561, the p type ZnO based semiconductor layer56 with the inclination of p type impurity concentration can becomposed.

Other Embodiments

The present invention has been described by the first to thirdembodiments and its modification, as a disclosure including associateddescription and drawings to be construed as illustrative, notrestrictive. With the disclosure, artisan might easily think upalternative embodiments, embodiment examples, or application techniques.

Thus, the present invention includes various embodiments etc. which havenot been described in the specification.

INDUSTRIAL APPLICABILITY

The epitaxial growth method, the epitaxial crystal structure and thesemiconductor device, the ZnO based semiconductor, and the fabricationmethod and fabricating apparatus of the ZnO based semiconductoraccording to the present invention are applicable to wide fields, suchas: light-emitting devices, such as LEDs or LDs (Laser Diodes) from ablue light wavelength region to an ultraviolet light wavelength region;a light-detecting element; a piezoelectric element; HEMT (High ElectronMobility Transistor); HBT (Hetero-junction Bipolar Transistor); and atransparent electrode.

REFERENCE SIGNS LIST

-   2, 12: Gaseous chlorine supplying unit;-   3, 13: Carrier gas supplying unit;-   4, 14: Source zone;-   5, 9: Heating unit;-   6: Water supplying unit;-   7: Carrier gas supplying unit;-   8: Growth zone;-   10: Substrate holding unit;-   11: Steam gas supplying unit;-   13: n type impurity material gas supply unit;-   15, 25: Group II metallic material;-   16: Growth substrate;-   17, 21: Gaseous chlorine supplying unit;-   18 a, 18 b: Gas supply line;-   20: Epitaxial crystal growth apparatus;-   22: First doping gas supplying unit;-   22 a, 22 b: Gas supply line;-   23: p type impurity material gas supply unit;-   24: Second doping gas supplying unit;-   40, 41: Substrate (sapphire, ZnO);-   40 a: Surface of substrate;-   40 b: Back side of substrate;-   42, 43: Buffer layer;-   44: ZnO facet;-   46, 47: ZnO layer (ZnO based semiconductor layer);-   52: n type ZnO based semiconductor layer;-   54: ZnO based semiconductor active layer;-   56: p type ZnO based semiconductor layer;-   60: p side electrode; and-   70: n side electrode.

The invention claimed is:
 1. A hetero epitaxial growth methodcomprising: forming a buffer layer in contact with a heterogeneoussubstrate, the heretogeneous substrate composed of a substrate selectedfrom the group consisting of an a-plane sapphire substrate, a siliconsubstrate, a SiC substrate, a GaAs substrate, a GaP substrate, and aGaN-based substrate; and performing crystal growth of a zinc oxide basedsemiconductor layer on the buffer layer using a halogenated group IImetal and an oxygen material, wherein the step of forming the bufferlayer includes the step of performing crystal growth of another zincoxide based semiconductor layer on the heterogeneous substrate using agroup II metal and an oxygen material, and zinc which is an elementalmetal without any bonding to another element is used as the group IImetal, wherein a reactant gas is zinc and water in the step of formingthe buffer layer, and a reactant gas is zinc chloride and water in thestep of performing the crystal growth of the zinc oxide basedsemiconductor layer, and wherein in the step of performing the crystalgrowth of the zinc oxide based semiconductor layer, a crystal growthtemperature is not less than 800 degrees C., and a supplying ratio VI/IIof a group VI element and a group II element is more than 1 and not morethan
 100. 2. The hetero epitaxial growth method according to claim 1,wherein in the step of performing the crystal growth of the zinc oxidebased semiconductor layer, the crystal growth temperature isapproximately 1000 degrees C.
 3. The hetero epitaxial growth methodaccording to claim 1, wherein in the step of performing the crystalgrowth of the zinc oxide based semiconductor layer, the supplying ratioVI/II of a group VI element and a group II element is approximately 20.